the mod Inmis Addon places a 3d backpack on the player's back, and when used together with FirstPersonModel, when swimming downwards the backpack appears on the screen

i have the same issue, hoping to find some way to hide things like backpack in first person

This is one line to fix for them ( https://github.com/Globox1997/InmisAddon/blob/1.20/src/main/java/net/inmisaddon/util/BackpackUtil.java#L47 check here if the body is visible. If not, don't render the backpack too). I did add enhanced features to better hide stuff attached to the player, but since the backpack is just rendered in space without translating to the body/checking that the body is visible, there is nothing more I can directly do.

Just noticed that I already mentioned that over on the issue Globox1997/InmisAddon#12 and it has been prioritized. Closing this issue here.