I can reproduce this as well on my dev instance, but only if the post being moved is a topic post (OP).
Repro:
1: Go to a topic post with existing reactions.
2: click the admin wrench and select the post
3: move to an existing topic
4: existing reactions disappear (except for hearts)
Screenshots:
Discourse 3.2.0.beta2-dev, desktop mode, Chrome, Ubuntu 22.04