@mori.luca, while native tab integration has been a feature for a while in macOS, it seems that there is a new option in Big Sur to control when the tabbing functionality is used. See System Preferences->General and double check the setting for Prefer tabs. When I set this to Always, I saw the behavior you’re describing. It looks like the default for me was in full screen.
Can you check your setting? You might need to force-quit your designers before changing and retrying.
I’ll submit an issue on our end to look at this. Based on some initial research, there might be a way to adjust whether an application respects the system defaults or chooses its own behavior (which appears to be what we need to do here).
EDIT: you might need to set this to never if you’re using the Designer in macOS full screen mode.