Hello! Please read.

The Supplementaries mod changes the mechanics and appearance of the Wandering Trader, like so:

As you can also see, these traders don't have backpacks. As such, it is impossible to unlock the Wandering Bag model.

Having this issue too, various wandering traders aren't spawning with a backpack. the vanilla wandering trader seems to be a viable spawn still but also doesn't get a backpack.

Yeah I've noticed this too and was thinking that it was just a bug on my part. Thank you for this!