Outlook keep resetting ribbon back to default after changes

You've hit the nail on the head. It's just another form of force-fed "advertising." Some idiot has decided that if we just get the icon in front of the user enough times, they'll just be compelled to try our newest BS features.

In my case, the customizations are restored when I click on each window (only for the window clicked). Give it a couple of minutes, and it will shift most or all of them back to defaults.

I have exported my ribbon configurations and reinstall them after MSoft installs what it wants to install. I have to do this perhaps once a month, maybe more frequently. Hard to say. Incredibly annoying. My fear is that MSoft's goal is to make the client and browser versions of Outlook have the same user interface....and in the process will wreck the client. I clicked on the "Try the new Outlook" button in the client, was disgusted with the result, and reverted back to (I guess we can call it) "Old Outlook." I've read on the web that for some it was difficult or impossible to switch back to "Old" so proceed with caution on that one.....

Sadly the "New Outlook" is a junker! It reminds me of the "olden days" and the way Yahoo and AOL's first browser-based email platforms looked. I don't know if MS is going for retro or their Gen-Z developers just think they're creating something new....

over the last few years there have been persistent rumors that MS ultimately wants to force users of the desktop version of Outlook to move to the web version, which they have more control over.

I have a love-hate relationship with Microsoft, while I hate them for their arrogance in assuming that only they know what is best for the user in this case I cannot agree with your assertion that "Outlook365 is ancient, and hasn't had any significant UI improvement in years". I have been using Oulook for nearly 30 years, while admittedly the basic UI has not changed much (maybe becuase it "just works"), but it has always been evolving and there have indeed been many changes and additions (including to the UI); some of them I like and use, others I don't use and disable. Until recent years when the the marketing dept began to take precedence over developers, many of the changes were in response to user feedback.

Nevertheless, the ever-changing ribbon really makes me angry because there is no way to disable it, somebody in marketing obviously clicked on the "Ignore User Feedback" box.