When testing the 4.18.0rc1 client on rpi4 (Raspbian 12), I was unable to send any system keys to the server when in fullscreen mode (alt+tab, super key etc). It looks like this feature is broken on the wayland window managers available in Raspbian (Wayfire & Labwc). Switching to X11 solved this issue. To switch window manager, run > sudo raspi-config and go to "6 Advanced Options > Wayland".
Note that the behaviour is identical using client 4.17.0, and I'm positive that keyboard grab worked when we did release testing for 4.17.0.