From here https://www.phpbb.com/community/viewtopic.php?p=14070741#p14070741

Post delete reason does not display if a post is first soft deleted and then permanently deleted. The reason will display if soft deleted and if permanently deleted without soft deleting it first.

I have verified this via my local test forum (see attached) and does not appear to affect soft deleting a topic and then permanently deleting a topic.