Prevent changing ticket secret of gift-card-issuing order positions (issue #6248)

src/pretix/base/secrets.py

@@ -245,6 +245,9 @@ def recv_classic(sender, **kwargs):
 
 
 def assign_ticket_secret(event, position, force_invalidate_if_revokation_list_used=False, force_invalidate=False, save=True):
+    if position.issued_gift_cards.exists():
+        return
+
     gen = event.ticket_secret_generator
     if gen.use_revocation_list and force_invalidate_if_revokation_list_used:
         force_invalidate = True