Same problem here!
Couchbase v6.6.2. I have enable_shared_bucket_access: true and Metadata Purge Interval: 1 day.
The sg_tombstones_x1 index counts 18mln items.
If I run this query to get the min tombstoned_at I get a date that is 27 days ago
SELECT RAW millis_to_str(1000*min(META(b).xattrs._sync.tombstoned_at),'1111-11-11')
FROM `my-bucket` AS b USE INDEX (sg_tombstones_x1 USING GSI)
If I manually run a compaction, nothing happens. Why?