When opening replay, all non-vanilla items and mobs are replaced to another random object.

As far as I'm aware, this is an issue with the original replay mod as well. It happens if after a replay, you change the loaded mods (or update and a mod changes content)

Can confirm this affects more than just changing the mods. I have recorded a session and then closed the game and reloaded it with my set of mods, for some reason it then changes every modded block and entity in the game to something else. This is a problematic issue with the mod.