Why do the icons of some apps like dbeaver and stremio not appear in the task switcher?
Welcome to the forum! 
It is possible that those applications somehow lack an icon in the icon theme that you’re using, in which case they get a generic one, but just to be sure, you could try the following command in a terminal window — as yourself; don’t use sudo… 
kbuildsycoca5 --noincremental
If it doesn’t work, then perhaps you can try using a different icon theme. 