Fix process mentions for local users, as local users are considered to use OStatus (#5618)

app/services/process_mentions_service.rb

@@ -17,7 +17,7 @@ class ProcessMentionsService < BaseService
         mentioned_account = nil
       end
 
-      next match if mentioned_account.nil? || (mentioned_account.ostatus? && status.stream_entry.hidden?)
+      next match if mentioned_account.nil? || (!mentioned_account.local? && mentioned_account.ostatus? && status.stream_entry.hidden?)
 
       mentioned_account.mentions.where(status: status).first_or_create(status: status)
       "@#{mentioned_account.acct}"