Topic to look for: https://www.phpbb.com/community/viewtopic.php?f=556&t=2479116
Problem: As recommended by mods community, I should resync the topics to fix wrong topic_first_post_id that are present in many of my topics, since I've made a lot of merges between topics. Those merges were made in phpBB 3.0 and 3.2, so I really don't know which of the merges are not taking in account the topic_first_post_id.
Because when merging 2 or more topics, it should fix the topic_first_post_id after the merge. (I still not check if thats happening at 3.2.2).
The problem here is when I want to fix this problem and I run the topic resync at ACP but it doesn't fix the topic_first_post_id.