Assuming you mean Couchbase Server here right?
Anyway, _exp is based on the ability to purge. Purging is not a replicated action, meaning that it is correct that it does not disappear in your local database. If you want it gone in your local database as well you will have to purge it there as well. PouchDB is not developed here so I don’t know its capabilities but can it set expirations too? Couchbase Lite in 1.x will react to the presence of the _exp property and setup its own local expiration. I don’t know what PouchDB does, if anything. If it doesn’t seem to be working, are you able to pull pull up the _exp data and set an expiration yourself?
At the end of the day PouchDB is not 100% compatible with Sync Gateway. Couchbase has made some moves away from and on top of the CouchDB protocol that PouchDB adheres to. A few months ago they merged a PR to remove all traces of Sync Gateway from their documentation (see here for more info) so take away from that what you will.