First I thought that this was a regression introduced by PR https://github.com/mantisbt/mantisbt/pull/1529, but the same behavior can be observed in 2.22.0 as well.

Nevermind what I wrote in 0026128:0062770 - it does NOT occur in 2.22.0, it is indeed a regression, introduced by MantisBT master 255dfdf2

When adding the attachment, the file_attach_files() function calls file_add() with $p_user_id = 0, while the function expects null to set the default user.

Data fix for attachments having user_id = 0 : set user_id to the linked issue's reporter, if the issue's date_submitted is the same as the attachment's file_added date

update mantis_bug_file_table f 
join mantis_bug_table b on f.bug_id = b.id 
set user_id = b.reporter_id 
where user_id = 0 and b.date_submitted = f.date_added;

PR https://github.com/mantisbt/mantisbt/pull/1562