Index: app/models/issue.rb
===================================================================
diff --git a/app/models/issue.rb b/app/models/issue.rb
--- a/app/models/issue.rb    (revision ec859e41d689ee3432113c3bd74ba48d060c9a2f)
+++ b/app/models/issue.rb    (date 1640777418920)
@@ -975,7 +975,7 @@
     return [] if project.nil?

     users = project.assignable_users(tracker).to_a
-    users << author if author && author.active?
+    users << author if author && author.active? && project.users.include?(author)
     if assigned_to_id_was.present? && assignee = Principal.find_by_id(assigned_to_id_was)
       users << assignee
     end