I can replicate with a new app:
Create a RibbonForm.
Make 2 tabs in the ribbon.
Add buttons to each. (Make sure they are different so you can tell you changed the tab)
Run the app.
If you are on tab 1 or 2 and then minimize the app...
When you restore back whatever tab you were on it is stuck on.
The tab strip in the ribbon switches but the contents of the ribbon are stuck on the tab you minimized on.
I have a paid subscription.
Am using the 2.6 beta.
Does this everytime.
I looked around the forum and found nothing referencing the issue.
Maybe this was part of the resize bug fix you did for the ribbon.