I have an account at outlook.com which is Microsoft free Exchange Server.
Most people use it for Web Mail.
I am using it from 2 computers (or more). I am using this scheme for years as synchronized personal categorized knowledge base and never had similar issue since 1998.
Problem started 6.9.2019 minus some days or 1 or 2 weeks ago.
Exchange Server version
I think version of faulty Exchange now is 15.20.2284.7 as seen in Outlook icon in down right corner - "Connection Status". I hope I read the correct one.
firstname . lastname @outlook.com
(removedID) is the same at both computers
The issue is related to Custom defined field in my custom form (all forms started to be affected):
To simulate the issue you start designing random form:
I used Task based form in all examples in my Task subfolder. I.e. MessageClass will be IPM.Task.SomethingYouPick
Note the "Amount Test" field. It is of the type "Currency". This and only this type has problems.. All other types, like Text, Number, Date... behave OK.
Before you start test, don't forget to Publish the form to your test folder:
In Folder Properties you have to tell that new items will be of your Form type:
Then you start creating some random items (in my case Tasks) of the custom form type.
We need to do it to create items with our customized "Currency" field.
You can use Filed Chooser to have a look like this:
Black box indicates the values are still ok at Computer1.
Now, press Shift + F9
It starts to Synchronize the Items, Form, Views to the Exchange Server. In our case outlook.com.
At this moment, after sync, you still see all values are ok at Computer 1.
Now get to the Computer 2.
Press Shift + F9 and you see all appears there
But look. The Symptom appeared. See Red box.
All other fields synced fine.
When we continue with test, we edit this field at Computer 2 to try if we can fix it.
We enter 1000 Kč as new value. We change some values like Subkect or the Size (of type Number).
You see we see it now correctly at Computer 2.
but see what happened after we sync it to Microsoft's Exchange.
And look back to Computer 1, there press Shift+F9 to propagate changes back from Exchange.
... and again you see the Value got lost. (You see the zero "0,00 Kč" below? it means empty value indeed). It re-wrote the old value instead of updating it as always. We expected 1000 Kč there and we see wrong value 0,00 Kč. And as you see e.g. in Subject or in Categories, which is multivalued field. Even Categories behaves ok.
But all text and number fields were synced correctly.
You see, corrupted data.
There is not even 0,00 Kč as seen in Form when you open the item because the value is indeed missing. The 0,00 is auto-offered by form because it thinks the item has here no value.
I have the following versions of Outlooks (English)
but also many others, including Outlook 2019.
This issue apparently is not in client, but in Server part. Microsoft upgraded their sever and I did not touch the clients. I have stable environment.
Also, as I tested other Exchange Server provider and repeated this test at him, problem is gone. Disappeared.
Also I have removed .ost files and synced all from scratch. So it is not corrupted ost files.
I tested more forms. All have the same problem.
I also cleared Forms cache everywhere. Did not help.
Also I tested various OSes in Virtual machines, various Outlook, various Mail profiles. Exchange as Primary account in profile, Exchange as secondary account in profile. Nothing like it matters. Problem stays.
When I move the folder to PST file, of course there is no issue. This is my temporary workaround of the problem. Because the issue gets invoked by syncing from Exchange.
I know the problem appeared somewhere between August and start of September.
I know 100% It cannot be a client issue.
The Currency is perhaps dependent on Regional Settings of Windows, which is set to Czech:
so in Hungary or Currency amount it will be Forints, in US $$$dollars. Which kind of explains, why using this kind of field is not popular and is perhaps obsolete because i has no sense to store value of the "current currency".
The problem manifests whenever you change something it the customized Task item (for example a Subject) and save and thus Exchange server detects necessity to synchronize the item based on some hash comparison. It does not change the value of existing items that were not changed, thus F9 (or Shift+F9) does not "touch" them.
Windows 7 64-bit edition (all English)
note that Outlook is always 32-bit
Sooner or later it will affect some business critical application or customer.
To replicate this issue you can have 1 single Virtual Machine, 1 single 32-bit Outlook and create two independent Mail profiles because they use separate .ost files, thus it must go via exchange sync. You have to Exit and start Outlook with Mail Profile 1 and then later with Profile 2... etc...
Exchange Server team at outlook.com providing department needs to check it.
Ticket at outlook.com: RE:  Approximately since September 2019 OutlookTicket from chat session: Reference number 1483783260
This is a kind of issue that corrupts data.If remote session is necessary I will be glad to demonstrate it live.
In case you want to skip Form Design, please download prepared PST file and Copy the Folder into you Exchange and you can test sync: