Long story short,
I have 2 keyboard layouts: EN, RU
Randomly selected keyboard layout change shortcut to “ALT while holding”
But now, I cannot change it to preferred “Alt+Shift”
Also Alt+Tab, Alt+F4 are not working, but that’s a different story
It is true that there are several key combinations that serve other things and are not valid, but in lubuntu 20.04 ALT + Shift works perfectly for me to change from English to Russian
Are you using another type of keyboard input like fcitx or similar that could cause this to fail? It is tested on 20.04 on my machine and I can change options without problem