I just went to my money exchange stall to collect the coins. I collected 150 tin coins just fine, but there was no "collect" button on my copper for cast slot. There should've been 120 cast coins, because I left 10 stacks of 10 copper coins there, each stack costing 12 cast coins.
I guess it might be a thief, but I didn't see any scents, despite walking away and back 3-4 times. It would also be very weird for a thief to just steal those 100 coins and leave 150 that are still there. Not to mention that none of my other stalls are missing anything.
The stalls are claimed by my alt East Hearthlands Trading Co, and the only character on his kin list is my main.