Yes this is a long-standing bug, maybe-probably-possibly-potentially-who-the-f-knows to do with the “catch up” mechanism that simulates time spent while you’re away without actually running every crafting station in real time (a good idea for server performance reasons).
In most cases, it’s not a problem, because worst case it gets you a bunch of materials faster than you otherwise would have: on my server we call it “rolling a 20 on crafting” or variants thereof.
The problem comes when the same crafting station that turns raw material into product also turns product back into raw material at a reduced rate every “conversion” loop. Simple solution to THAT problem would be to move coin production from the Smelter/forge to the Blacksmith or (my choice) Artisan’s table. It’s been suggested quite a few times by me and others, but no-one seems to care. Mainly because it can be worked around fairly easily unless you’re ALSO affected by the insta-craft bug.
Simplest solution is to not bother with the coin mold at all, it’s not like coins are that useful and you probably find plenty in chests and other loot for the few cases where it IS useful (YMMV if you play modded obviously).