Why Postgres query is not using column in Index Condition but uses it in Filter condition

I am running a simple join query, where the table incident_tracker_details is covered by a suitable index. The query is slow because it doesn`t use all the columns in Index Condition, but does use the column as a Filter leading to a large heap searches


Query:
SELECT
relevant_incidents_per_inventory_item.inventory_id,
tracker_id
FROM
relevant_incidents_per_inventory_item
JOIN
incident_tracker_details
ON
incident_tracker_details.incident_id = relevant_incidents_per_inventory_item.incident_id
WHERE
incident_tracker_details.company_id = :companyId
and incident_tracker_details.division_id = :divisionId
AND incident_tracker_details.value > 0



the index in use:
create index companyid_divisionId_incidentId_idx
on incident_tracker_details (company_id, division_id, incident_id)
where (value > 0) include (tracker_id);



the relevant_incidents_per_inventory_item does not have any relevant indexes and the table is very small (few hundreds of rows).


the actual query plan:
QUERY PLAN
Hash Join (cost=6901.58..6913.72 rows=1 width=8) (actual time=90.663..315.278 rows=522 loops=1)
Hash Cond: (incident_tracker_details.incident_id = relevant_incidents_per_inventory_item.incident_id)
-> Index Only Scan using companyid_divisionId_incidentId_idx on incident_tracker_details (cost=0.57..12.68 rows=6 width=16) (actual time=0.039..202.787 rows=246774 loops=1)
Index Cond: ((company_id = '777704491747470679'::bigint) AND (division_id = '777795770460846005'::bigint))
Heap Fetches: 124039
-> Hash (cost=6900.88..6900.88 rows=10 width=8) (actual time=86.704..86.709 rows=259 loops=1)
...not relevant part
Planning Time: 0.518 ms
Execution Time: 315.402 ms



Why is the "incident_id" not part of the Index Cond? this leads to retrieving all the rows under companyId, divisionId , and then the engine has to filter all the rows for those having incident_tracker_details.incident_id = relevant_incidents_per_inventory_item.incident_id


I expected to see all the columns used in the Index Condition, leading to fast retrieval of only the relevant data