I've already posted this to Supplementaries but figured I should post it here too.
To replicate this, you need a cage from Supplementaries, a goblin/fiery goblin from Goblin Traders, and a friend to do this with.
(For this example, I will be using fiery goblins, and players A and B. Fiery goblins can trade you one emerald per carrot given with a max of around 24 I think.)
Player A has to right click on the fiery goblin to begin trading with it, and player B has to pick up the fiery goblin with their cage. Once the fiery goblin has been picked up, player A can trade the max amount of carrots and the trade will be locked off. Once player B places the fiery goblin on the ground again, the trade will not be locked off and they can redo the trades over and over. The same thing can be done with other trades.
I used goblintraders-1.8.0-1.19.2 in BMC3 to find this btw

Was this ever fixed?

Thanks for the report. I'll look into publishing a fix soon.

It probably was "fixed" in a way that just made things worse. Now you just can't release a goblin from the cages in general. Ruined 2 good mods because of 1 wacky bug.