How do I stop Windows 10 from Resetting default file associations?

Does it happen for all file types / applications or for a particular file type / application?

A. What is your computer's make and model? For example Lenovo T470

B. What is the Windows version and build? (To check this, type winver in Windows Start Search box and click winver)

C. Did this problem start after any Windows update (To check Windows updates, Windows Key+I > Update & security > View Installed update history)

D. What troubleshooting steps have you already undertaken?

This appears to be a "new feature" in Win10 that is running Wild. MS is treating user set defaults as "malware attacks" and "improving' them back to MS defaults ... <sigh>.

Here are a couple of potential fixes

WIN10 – HOW TO CONFIGURE FILE ASSOCIATIONS FOR IT PROS? – DEFAULT FILE ASSOCIATION
https://blogs.technet.microsoft.com/windowsinte...
Hello, my name is Ismaël Limbada and I am a Support Escalation Engineer for Windows Core Team. This blog post is about file association configuration in Windows 10. After reading it, you will be able to configure file associations in Windows 10 and avoid this notification: An app default was reset.
Dism.exe /online /export-defaultappassociations:”C:\Users\Luser\Desktop\CustomFileAssoc.xml”

Discussion about Killing Edge, sorry no can do … <big sigh>
It is a rerun of the IE vs Netscape ware. Once again MS tried to make the Edge application “integral” to the OS, like they tried to do with IE years ago. I have read that the PowerShell remove-appxprovisionedpackage command will not work specifically on Edge.
So Edge is like a rootkit malware that we just can’t get rid of.
Setting something else to be default browser also may or may not work. MS has “improved protection” for your file default. If changes are not made in the “proper” way, Windows 10 will reject the changes and automagically “protect” you by setting them to the system default, typically Edge. AHHHHH!!!!!!
This next article goes into painful, technical, detail on the process. It suggests using the built in <cr>applet to set default file associations. So I checked, and in 2 of my personal problem cases, the <cr>applets say my preferred application is set as the default for the file type <oh goody!>. But regardless of what the <cr>applet say, that expletive deleted EDGE opens instead ... <big sigh>
https://blogs.technet.microsoft.com/windowsinte...
Bottom line, Window 10 “improved” default file associations. In the process it broke them <to MS’s “advantage”, golly gee, what a surprise> . And there does not appear to be anything we can do to fix or work around this problem at this time.
Touch keyboard is auto-launching unexpectedly on the log in screen after system restart – Insider Feedback
With Build 17017, we’ve fixed the issue. Thank you for your feedback and patience while we investigated the issue. Please make sure you’ve updated to the latest Insider build and submit a new item in Feedback Hub app if you feel this feedback hasn’t been fully addressed.

WIN10 DEFAULT FILE ASSOCIATION PROBLEM
I think you are being caught by an 'improved' MS 'feature'. It is supposed to prevent malware from making changes to Windoze. But it is running out of control. Sometimes it does not recognize when a user feels a 3rd party tool is better than MS provided defaults. I ran into something similar with 1709. It kept "improving" my browser from FireFox to Edge. Sorry, I don't remember how I beat it into submission.

I have seen a couple of comments where people say that logging in to an admin account on the PC and the making the changes to Defaults has worked for them.

This article, and it's comments suggest a couple of possible fixes, including registry hacks:
https://winaero.com/blog/prevent-windows-10-fro...


This article repeats the suggestion for the registry hack, and includes a link to a 3rd party tool (I have not tried it) that automates the hack:
https://superuser.com/questions/1022063/windows...

The potential problem is that these tips were written a year or two ago so may not apply to Win 10 ...

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Standard boilerplate legalistic bafflegab disclaimer for any links to non-Microsoft websites:

The link(s) to non MS-page(s) appears to be providing accurate, safe information. I suggest you stick with the “manual” instructions. Unless I explicitly suggest it, avoid downloading their “utilities”. Thoroughly research any product advertised on the site before you decide to download and install it. Sometimes they include 3rd party programs, "unwanted" programs that are automatically installed during a standard install. So, if offered, do a "custom" install and carefully read each step. One of the steps MAY give you an option to install, or NOT, "additional" programs. 99.9% of the time you don't want to let them be installed. For example, when you install Adobe Player you are given the option of installing 2 additional "free trial" programs: Potentially Unwanted Programs: MacAfee Security Scan and MacAfee Safe Connect. The option to install is turned on, you have to manually turn it off. Those are PUP's. If you click "OK"/"Next" without paying attention you get the 2 "extra" programs.

As well, watch out for ads on the site. They may advertise products that include PUPs. Thoroughly research any product advertised on the site before you decide to download and install it.

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Today Widows 10 decided to usurp my file associations for 6 different extensions.  Furthermore, it decided to assign to them apps which I have never actually opened.  Again.  The only reason I've continued to use Windows for the last 2 years is a 3rd-party video game that requires it.  The game has been beaten.  It's time to move on.

"have seen a couple of comments where people say that logging in to an admin account on the PC and the making the changes to Defaults has worked for them.

This article, and it's comments suggest a couple of possible fixes, including registry hacks:
https://winaero.com/blog/prevent-windows-10-fro...


This article repeats the suggestion for the registry hack, and includes a link to a 3rd party tool (I have not tried it) that automates the hack:
https://superuser.com/questions/1022063/windows...   "

So we need hacks to use our programs instead of microsoft's ?

How is that even legal ?