I’m using X. The Plasma version is 5.27.11, from the kubuntu-backports repositories (I’m using PopOS, which is based on the Ubuntu 22.04 LTS, and their native Plasma lacks some of the features I wanted).
I am using the US-Colemak layout. All my reports of keybinds here are relative to that layout; e.g. Alt-D is pressing the physical keys that would be Alt-G under QWERTY.
I’ve found several other Alt- keybinds that get shadowed. Somewhat exhaustive list: Alt-G, Alt-R, Alt-D, Alt-H, Alt-| aka Alt-Shift-\, Alt-Shift-X, Alt-Shift-B, Alt-Shift-K, Alt-Shift-M, Alt-< aka Alt-Shift-,, Alt-> aka Alt-Shift-., Alt-? aka Alt-Shift-/. It may be notable that the entire right half of the bottom row is shadowed under Alt-Shift.
I’ve verified that these keys all work in a VT, so it shouldn’t be any kind of hardware issue.