I just got this problem after manually updating my Laptop to Linux Mint 20. Before this I used Ibus together with Eurkey without any problem. After the update I realized they both didn't work anymore. So I removed ibus and tried Fcitx. Got it to work but Eurkey still doesn't stay. When I boot up I have to enter the command "setxkbmap eu" for the Eurkey layout to work. And as soon as I use Ctrl + Space to change the input method and then back Eurkey layout is reset to English US layout. I haven't tried a fresh install of Linux Mint 20 yet so I'm not sure if that's the solution and even then I don't want to reinstall everything..