I just checked on Linux (Thunderbird 128.5.2esr, Opensuse Tumbleweed) and the behaviour is the same.
If I search “PMASUP236”, it returns the email as a result.
If I search “SUP236” it does not.
This is using the normal search function (top of screen in current version). Quick Filter does not look at attachments at all by the looks. The “Attachments” toggle is only a has / does not have attachment filter.
I used to like Ubuntu LTS because it was just Debian that wasn’t quite as out of date, but more recent installs seem to suggest that you only get all the patches if you subscribe to their paid service? Not sure what the fine print is on that.
This box was turned on, Nextcloud installed, and never touched since (side from apt updates).
06:49:18 up 2081 days, 22:07, 1 user, load average: 0.21, 0.33, 0.42