PowerDevil gets the same listing of displays from KWin, namely
{"kwin:eDP-1", "kwin:HDMI-A-1"}
That's expected, KWin can still do software brightness, even if it doesn't have control over the backlight.
Does KWin need to remove the external brightness controls coming from PowerDevil when the process disappears for whatever reason (crash or otherwise)?
libwayland automatically calls destroy on all Wayland objects when the process disconnects, there's no need to do anything additional to cover that scenario. It should only be possible for a brightness device to stick around if powerdevil is still running and hasn't destroyed it yet.
Is the issue consistently reproducible? If so, a WAYLAND_DEBUG=1 log of powerdevil would be useful