fix: document naming rule not working for subscription invoices (#27386)

diff --git a/erpnext/accounts/doctype/subscription/subscription.py b/erpnext/accounts/doctype/subscription/subscription.py
index 445eb3c..8171b3b 100644
--- a/erpnext/accounts/doctype/subscription/subscription.py
+++ b/erpnext/accounts/doctype/subscription/subscription.py
@@ -400,6 +400,7 @@
 
 		invoice.flags.ignore_mandatory = True
 
+		invoice.set_missing_values()
 		invoice.save()
 
 		if self.submit_invoice: