This is part of T79273 where separating a stroke would result in
multiuser gpencil data for the original object.
Real underlying issue seems to be that gpencil multiuser objects (also
the ones created by Alt+D duplicating) have a problem evaluating
modifiers correctly [this will need further investigation].
Not sure if this is a limitation of D5470?

This patch only corrects the usercount on separating [which already fixes
the scenario reported because singleuser gpencil modifiers work
correctly].

Note: we could have also called ED_object_add_duplicate with the
USER_DUP_GPENCIL dupflag -- that would have taken care of the usercount
--, but then the whole following logic in gpencil_stroke_separate_exec
would need to change from adding layers/frames/strokes to
removing these.

Part of T79273