I have this same issue, except that my customized settings will not even stay for 10 minutes! As soon as I click on "reply" or 'new email" then back to home I have lost my customizations. Then I go from home back to "reply" and my settings there are gone and I have all this OneNote, Viva Insights, Templates, Polls and Immersive GARBAGE shoved back in my face. I do realize that all of these things help MS collect user info (for their own marketing reasons) but it's really annoying that as much as this software costs my company (with 100s of employees using MS 365) it isn't even easy or productive for us to use it. How about actually making products FOR THE USER instead of your own data gathering.
You might try manually disabling Office Updates (File/Office Account/Microsoft 365 and Office Updates/Update Options). For me this has seemed to reduce the frequency ribbon of icons being pushed back to the ribbon, but hasn't gotten solved the problem completely. While this is better than nothing like most workarounds it is a nuisance.
If you do try this don't forget to re-enable Office updates from time-to-time, occasionally there are some that you actually need, especially security updates. I re-enable Office updates once a month just before the first Tuesday of the month when non-security updates are released; after updating I immediately disable them. That way I have a whole month of quiet while Microsoft is busy releasing the inevitable patches for buggy updates. I use the same procedure for Windows updates as well.
Apparently Microsoft releases non-security updates on the first Tuesday of the month and security updates on the second Tuesday. For Windows I update everything just before the first Tuesday of the month.