How do I reclaim space in filestream data files for database in a SQL Server Availability Groups
I believe you are experiencing a sightly different scenario related to the bug described here, and officially reported here.

When a filestream table is dropped (or in your case rebuilt &amp; moved), garbage collection will not clean up if the database is in an AG. 

To work around the bug, you would need to:


Remove the database from the AG
take a log backup
manually run garbage collection
apply log backup(s) to secondary replicas WITH NORECOVERY
rejoin the database to the AG (by applying log backups, you can avoid a full resync)