Hello,
this problem may not be related to KiCad but to Linux Mint (21.2) but I hope someone here might be able offer an explanation:
In the PCB-Editor I can define my own layer sets (works) and I can select them with the SHIFT-TAB popup menu (works) but whatever I select the layer set does not change.
Very rarely it works for just one time but I can not find out why. 
There seems to be no usage of SHIFT-TAB by Mint !?
This happens since I switched from KiCad 7 to 8.07 and 8.08
Its not a major concern for me but I would like to use the custom layer set anyway 
Greetings from chris9 !