ORDER BY + LIMIT is a query which sometimes has issues, you can probably find more by searching. The planner thinks it'll hit the LIMIT pretty soon and only run a fraction of the index scan - but then it turns out to be wrong.
You might have poor statistics on project_id and/or tags. This *might* help: ALTER TABLE subscriptions ALTER project_id SET STATISTICS 2000; ANALYZE subscriptions;
But I'm guessing there's correlation between the two, which the planner doesn't know. If you're running at least v10, I'm guessing it would help to CREATE STATISTICS on those columns (and analyze).