Relind the way to reproduce, always :)
Add screenshots if you can

Way to reproduce:

1. Go to Theme management
2. Click on Extend button on Fruity
3. Click ok (or rename if needed)
4. Click close button
5. Click on Theme option at extend_fruity
6. See the difference : one tab for all

I didn't test on Survey theme option.

Capture d’écran du 2020-09-16 08-14-44.png (79,803 bytes)   

Capture d’écran du 2020-09-16 08-14-44.png (79,803 bytes)   

Relind the way to reproduce, always :) Add screenshots if you can

Thanks, I'll try to remember that. For this one it's too late, I can not modify issue after creating it.

No problem : i add it after :)

Thanks for report :)
I will mark it with low priority since there are other blocking issues atm. Anyway, it landed in our backlog.

https://github.com/LimeSurvey/LimeSurvey/pull/1674

When the theme config is copied, the engine node is removed (apparently it caused issues before).
Since the optionspage config is inside the engine node, it's removed from the extended theme config, and then the options.twig is used.
One option is to move the optionspage out of the engine node, so it's not removed.
Other option is to remove the engine node contents EXCEPT optionspage, instead of the whole engine node. This option seems easier and it was the one implemented.

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=30781