There exists a bug that seems to occur whenever you place items or some other thing where the item the player is holding disappears in third person pov. I'm not too sure whether this is an incompatibility between the mod First-Person Model or if it's a problem with this mod's code itself but I just wanted to report this ##problem.

tr7zw/FirstPersonModel#481 (comment)

I don't think this is related to either first person or not enough animations, as this would be reported way more then, and I haven't seen it myself. What's the Mc version, other mods, etc.

I'm afraid I don't remember the other mods on the modpack but I do know this is from Minecraft version 1.21, I'll go check later on what the other mods were but this is all I can say for now.

So this is the list I can get: