postgresql – Multicolumn GIN index not fully facilitated by query planner

I am experiencing unexpected behaviour with ProstgreSQL 11.10 query planner with multicolumn GIN index usage.

After defining an index on two columns (a,b) of a large table and executing a query involving conditions for both columns a = 4 and b @> '{"key": "value"}'::jsonb, the planner applies the multicolumn index only for one of the two conditions. It seems that another scan is executed in parallel for the other condition, using another index. Why can this happen?

A few more details:

The index is:

CREATE INDEX coupons_campaignid_attrs_index ON coupons USING gin (campaignid,attributes);

The query is:

explain (ANALYZE, COSTS)
SELECT COUNT(*)
FROM coupons
WHERE campaignid = 97 AND attributes @> '{"CountryId": 3}';

The plan is:

Aggregate  (cost=34532.02..34532.03 rows=1 width=8) (actual time=56613.161..56613.164 rows=1 loops=1)
  ->  Bitmap Heap Scan on coupons  (cost=32208.05..34526.31 rows=2284 width=0) (actual time=23541.180..56384.603 rows=1090012 loops=1)
"        Recheck Cond: ((attributes @> '{""CountryId"": 3}'::jsonb) AND (campaignid = 97))"
        Rows Removed by Index Recheck: 10532325
        Heap Blocks: exact=138539 lossy=583394
        ->  BitmapAnd  (cost=32208.05..32208.05 rows=2284 width=0) (actual time=23478.234..23478.236 rows=0 loops=1)
              ->  Bitmap Index Scan on coupons_campaignid_attrs_index  (cost=0.00..1543.35 rows=179379 width=0) (actual time=14247.668..14247.668 rows=81803572 loops=1)
"                    Index Cond: (attributes @> '{""CountryId"": 3}'::jsonb)"
              ->  Bitmap Index Scan on campaignid_deleted_idx  (cost=0.00..30663.31 rows=2284099 width=0) (actual time=8658.669..8658.670 rows=3097095 loops=1)
                    Index Cond: (campaignid = 97)
Planning Time: 1.711 ms
Execution Time: 56619.880 ms