I’m also trying to solve it from here, I think I found the solution.
In system settings under “dock” there is a another option, " open documents in tabs" with a pull down menu for " always" , “only in full screen” and “never”
it was on always (sounds logic) now I placed it on “only in full screen” and it works like is should.
As I tough it needed to be a setting as it works on my other iMac and also no one else had this problem.
thank you in advance for your feedback