Hello, please can you provide the XAML code too?

Can you try without running the task manually to see if Windows still regularly triggers the task ?

On which Win10 version was the default associations configuration file generated ?
Can you try to create the default associations file from Windows 10 1909, don't change anything and deploy it on a test computer to see if it works as-is ?

Have you checked that no program tries to change file associations on first logon, for example ? Just to be sure that the issue is not caused by something else...

Additionnally, can you confirm that you proceed as follows: change the default association on a "reference" computer using the Settings app (GUI), then generate the file with Dism /Online /Export-DefaultAppAssociations:c:\path\AppAssoc.xml and that you don't edit the file manually afterwards ?

The problem appears again after gpupdate /force followed by a logoff/logon ? Or does it only happens once per profile?
Did you use /Import-DefaultAppAssociations with Dism or just the GPO settings to import the file on the computers ?