I am not able to reproduce this.
I put a long text in Notepad, copied from there and pasted in Excel once in a cell and once in formula bar. But it didn't behave like the way you are describing.
Have you experienced this problem only in Excel 2016 or in previous versions also?
You can try some general troubleshooting steps for Excel and see whether this corrects the problem or not
1. Try repairing your office installation. irst try Quick Repair (takes 5 - 10 mins) , if that doesn't help then try Online Repair (Takes more than 30 mins but less than 1 hour in general)
https://support.office.com/en-us/article/Repair...
2. Please look into C:\Users\User_Name\AppData\Roaming\Microsoft\Excel (replace user name with your user name, you can directly reach here by copy and pasting following without quotes in Explorer address bar - "%appdata%\Microsoft\Excel") and see if is contains few files / folders. Move those files to some other location (i.e create a backup of those files / folders and delete all files / folders from here). Hence, make Excel folder blank.
Now open Excel and see if the problem disappears or not.
Note - If the above path is not applicable for your installation, you need to find XLSTART path. Excel folder contains XLSTART folder in itself (In case, your XLSTART path is not in Appdata)
1. Open Excel
2. ALT+F11
3. If Immediate Window is not visible, type CTRL+G. Same can be also be accessed through View > Immediate Window
4. Type "? application.StartupPath" without quotes and press enter
5. You will have your path below.
3. Sometimes, add-ins can cause problems. To determine, if add-in is a problem start your application in safe mode and see if problem has gone away.
To start in Safe Mode -
Hold CTRL key and click on application icon and don't release the CTRL key till it asks you for Safe mode confirmation.
If yes, then start application normally and disable add-ins one by one and start application again every time you disable an add-in to determine the culprit add-in.
To disable add-in > File > Options > Add-ins > Down below you have Manage: Com Add-in : Press Go > Untick the Add-ins to disable and tick to enable them
4. Try to login through another Windows profile. If there is some corruption in profile which is causing this problem, this would correct.