Caused by 2eeec49640.
Above commit would early out when falling through the specialized
greasepencil selection operator to view3d_select_exec. But in order to
select posebones in grease pencil weightpaint mode, we still have to
continue with view3d_select_exec.
Now check this special case [with convenient
`BKE_object_pose_armature_get_with_wpaint_check`] and DONT early out in
that case.
Should go into 3.3 LTS as well.
Philipp Oeser