Unfortunately, this seems to be a bug. I have Tab S5e with OneUI 3.1 (Android 11) and I need to change my physical keyboard (Universal Microsoft Keyboard) to Slovak layout. But settings are not followed, even though I set up the language of physical keyboard correctly. I have been doing this for several years, setting up different devices with different versions od Android, beginning from version 4.4.2. So I believe I can positively confirm this is a bug.
I also tried to restart tablet, unpair and pair keyboard again. I tried Samsung keyboard and Swiftkey, I removed Slovak and English language from Swiftkey and then installed it back. I tried different combinations (have only Slovak, then only English, then both, delete layout, reinstall layout). Nothing helped. When I use Samsung keyboard and use keyboard shortcuts to switch layouts, I see toast notification saying the layout changed to English/Slovak, but in reality, I am always stuck on English. 😞