So I tried again, but I made sure there were only the 3 cow pelts and the bag of lime, since I thought the steak might have caused it to get stuck tanning the first time. The tanning process finished, but everything in the barrel vanished.

The problem did persist upon relog. I relogged a couple of times and it was still stuck.

I just realized the dirt bike helmet was located further down in my inventory. I can remove it after all. I'm an utter idiot. I'm sorry for wasting anyone's time.