I'm not sure I've ever heard of anyone having this problem aside from that large server update a year or two ago and their "desktop experience". Can anyone tell me more?
However, you can get the impression it happens more regularly because when individual default apps, like Photos, get updated (or if there is a problem in the registry) they may reset the association for the files they expect to handle.
[0] https://wccftech.com/windows-10-bug-unable-apps-defaults/
[1] https://support.microsoft.com/en-us/help/4467682/windows-10-...
"A customer reported that each time they restart their Windows 10 PC, the file association for PDF documents keeps getting reset to the default, which is Microsoft Edge. [..]"
I guess there is no way (given the current registry design) to differentiate between the user modifying the registry and a program modifying the registry.
Again, coming back to the original message - the case scenario is not that some program is changing the preferences. It the user changing the setting, and Win10 silently discarding the user input (after a reboot or some automated update).