Despite the fact that some Linux distributions have moved over to Wayland as default display server, most GUI software still uses X11 (XWayland) by default, and that includes Qt 5.12. This makes Squish (and the AUT) appear like they are running on x11. Most things work, but grabWidget() for some types do not.
Enforcing pure Wayland mode is possible on some platforms, through the use of environment variables. Using these with Squish can also cause problems. Currently Squish still uses native X11 APIs in some places to make some of its AUT interactions possible.