Why do my items keep getting stolen by a Jollin everytime I try to throw an item into the bonfire? I don't want to keep throwing things in there if this is going to keep happening.

How many times have you tried? The first three items I tossed were stolen, but that's it. I haven't seen him since. The third time, the text made it seem like the thief is an unlockable battle opponent.

It is an unlockable battle opponent that's only available during Fireside. I guess they're still having some issues with battling and stuff though, because the Event tab is still missing from the battle coliseum where the challenger would usually show up.

Yeah I was thinking that might be the case. I've only done it 3 times before but I'll go ahead and give it a shot now. Thanks guys!

... There's a Jollin thief?

He doesn't show up after you unlock him as a battle challenger, including if you've unlocked him in a previous year. His challenger name is Bandit if you've fought him since the last coliseum reset, but he's not available to fight yet this year since the event challengers tab is down

I went to check and apparently, I'm missing an entire tab of challengers in the coliseum? Huh. But thanks for the info, my brain must have completely imploded where that was concerned.