BUG #16895: Multicolumn GiST index returns incomplete results

The following bug has been logged on the website:

Bug reference: 16895
Logged by: Dan Bărbulescu
Email address: dan(dot)barbulescu(at)hollo(dot)software
PostgreSQL version: 13.2
Operating system: Debian GNU/Linux 10 (buster)
Description:

CREATE EXTENSION
IF NOT EXISTS
ltree;

CREATE EXTENSION
IF NOT EXISTS
btree_gist;

CREATE TABLE
test
AS
SELECT
_ ::TEXT ::ltree AS path,
_ ::bit(4) AS flags
FROM
GENERATE_SERIES(1, 1024000) _;

-- A large-ish table is needed, the results don't usually vary by a lot

SELECT
flags,
COUNT(*)
FROM
test
GROUP BY
1
ORDER BY
1;

-- The values are evenly distributed

CREATE INDEX ON
test
USING
gist(path, flags);

-- The "offending" index

SELECT
COUNT(*)
FROM
test
WHERE
flags = '0000';

-- Reindexing sometimes produces different results. This is as much as I've
been able to narrow it down, I haven't been able to reproduce with other
data types (i.e. text instead of ltree) or shorter bit strings.