I almost went back and edited my reply, because, yes, it is a design decision and I mis-spoke. Sorry, I should have changed it.
I did find there’s an option to stop the tab bar area from changing color and that makes it a lot better for me.
Oh, I also found just now that Bartender has a beta to improve compatibility by disabling the broken bits, with a fix coming later. It’s just that things changed under the hood so totally understandable, but it’s unusable for me right now because the icons gradually shift to the left of the screen! Comical in a way…, but the beta doesn’t seem available on Setapp. Just when I’d got it working happily for me again, too 