BUG #15861: jsonb exists query retuning inconsistent results

The following bug has been logged on the website:

Bug reference: 15861
Logged by: RJ Ewing
Email address: rj(at)rjewing(dot)com
PostgreSQL version: 11.3
Operating system: ubuntu 18.04
Description:

When running the following query:

select count(*) from network_1.sample_photo where data ? 'photoID';

I am receiving inconsistent result that are different every time I run the
query. Somewhere in the 16k range. However I know for a fact that every row
in the table has a 'photoID' key. There are 103k rows.

The following query works correctly every time:

select count(*) from network_1.sample_photo where not data ?
data->>'photoID' is not null;

When running explain, the later is performing a seq scan, where as the
former has the following plan:

Aggregate (cost=409.55..409.56 rows=1 width=8) (actual time=44.498..44.499
rows=1 loops=1)
-> Bitmap Heap Scan on sample_photo (cost=20.80..409.29 rows=103
width=0) (actual time=7.008..35.916 rows=16431 loops=1)
Recheck Cond: (data ? 'photoID'::text)
Heap Blocks: exact=9932
-> Bitmap Index Scan on idx_network_1_sample_photo_data
(cost=0.00..20.77 rows=103 width=0) (actual time=5.273..5.274 rows=16431
loops=1)
Index Cond: (data ? 'photoID'::text)

The index "idx_network_1_sample_photo_data" is a gin index.

I am not able to reproduce this on my local machine. This problem happened
in version 10, and I just upgraded to 11 without any improvement. I tried a
vaccum full analyze & to rebuild the table indexes.

Is this a bug? Or am I missing something?

Thanks in advance