As reported here: https://www.phpbb.com/community/viewtopic.php?f=556&t=2479116

If two topics are merged, or if the first post is deleted the 1st post info will have wrong information, that we can spot easely if we use [Smart Subject Ext|https://www.phpbb.com/customise/db/extension/smart_subjects/].

I only recommend to use [Smart Subject Ext|https://www.phpbb.com/customise/db/extension/smart_subjects/] to be easier to spot the bug orelse you'll need to check it by looking directly to the database, and would not be so easy to spot the problem.

And as discuss on the forum, one solution would be to implement on the ACP - Forums - Syncing function, the ability to handle first post info. It should be done in any case.
That way we would have a simple way to fix the problem.

Aside that it would be great to fix the problem per se, when deleting the 1st post message, the topic_first_post_id should be updated to the next post on the topic, and when we merge 2 topics, the current 1st post id should go into the topic_first_post_id of the new topic.