MC: 1.12.2
ModVersion: 3.5.7
Forge 14.23.5.2847

Happens both in singleplayer and multiplayer
Tested with just betweenlands on a clean instance
Crashlog: N/A

When drag clicking items into a FUELED dual sulfur furnace, you will get kicked out of the GUI and the inventory will be cleared.

Reproduction steps:

1. place a Dual Sulfur Furnace.
2. place some sort of fuel in the fuel slots.
3. click drag something to be smelted between the two valid destination slots.
4. get kicked out of the GUI and lose all contents of the furnace.

Expected:
Stack should split and smelt properly.

Actual:
furnace inventory wiped.

Video: https://youtu.be/DgUjX8JxBI0

Good catch, thanks for the report.