I don't know if it would be relevant to this problem, but you are missing almost 1 full year of bug fixes. 11.4 was released on 20 June last year. Upgrading minor versions asap is recommended.
I do see this in the release notes from 11.8 last month ( https://www.postgresql.org/docs/release/11.8/)- Ensure that a replication slot's io_in_progress_lock is released in failure code paths (Pavan Deolasee) --This could result in a walsender later becoming stuck waiting for the lock.
