[FTS] Level-compaction: stat 'items_remaining' is incorrect

Build
5.0.0-3217

Level compaction turned on. Pls note, this stat is 0 at the end of the test with full compaction.
Consistently reproducible with

Testcase
./testrunner -i INI_FILE.ini -p skip-cleanup=True,cluster=D+F:F,level_compaction=True -t fts.stable_topology_fts.StableTopFTS.index_query_custom_mapping,items=1000,custom_map=True,cm_id=3,num_queries=100,compare_es=True,update=True,delete=True,GROUP=P0

Steps
1. Load 1k docs from EMP dataset
2. Create a custom index with level compaction on, wait for indexing to complete
3. Do some deletes (300). Check if bucket_count=index_count=es_endex_count=700.

Check the stat 'items_remaining' in the fts stats panel. You will see it stuck at a non-zero value.