BUG #17671: Query returns unexpected result

The following bug has been logged on the website:

Bug reference:      17671
Logged by:          Nikola Mirkov
Email address:      niko@aku.mx
PostgreSQL version: 12.12
Operating system:   Ubuntu 12.12-0ubuntu0.20.
Description:        

Hey guys,
When I run the following query:
SELECT foo, bar
FROM table1
WHERE fk_id = 637
AND utc > '2022-10-16'
UNION
SELECT foo, bar
FROM table2
WHERE fk_id = 637
AND utc > '2022-10-16'

I'm getting only one row per table. If I add the column "utc" to the list in
SELECT, I get the expected result.

Why is this happening?

Thank you so much!
Niko