OK, so this was the reply.
Hi Alan — based on our official info, this does look consistent with preset state not being saved automatically, rather than me being able to confirm a documented 3.9.1 bug.
What we do have says plugin settings and the project/plugin state are saved separately. If you edit a factory preset and don’t use the plug-in’s own Save control to create a new saved state, it can revert to the original preset later.
So please try this quick check:
load the factory preset
make your edits
in the plug-in window, click Save and create a new user preset/state
then assign/store that as a Favorite
reload the Favorite and test
If it still recalls the default preset after that, then it’s reasonable to treat it as a possible bug/issue in 3.9.1.
And I'm pleased to report IT WORKS!
If you edit a plug-in, then save it using the plug-ins 'SAVE' tab (I have created a folder in my USER space on my XL which is designated [Presets / plug-in name / preset name]. Then save this preset to one of 'FAVORITE' blocks. When you open a new project, the FAVORITE block you saved is available with all your edits.