When in multiplayer, the backpacks of other players render glitchily in the position that they put them on in and don't disappear until the player takes the backpack off.

Screenshot:

This screenshot shows that the object is indeed a backpack, as the shape is the same:

IronBackpacks version 1.8.9-2.0.5

Version please?

Ah yes, it's version 1.8.9-2.0.5.

A quick workaround could be just to make it invisible to other players for the time being

Yeah, I'm working on internally changing how the backpack is rendered, from it being an entity to just an extra model on the player. This is a decently large change (and I am swamped with schoolwork at the current moment) so it is taking a while, but I will eventually fix it.