When using the external heater to power vanilla furnaces, if the power is lost midway through a smelt there is a chance the item will be lost, and the furnace will be bugged to always show the "on" texture on it's front face.

Setup:
3 water wheels on a kinetic dynamo (tried for maximum power)
1 water pump providing for 1 garden cloche (if this matters it's config'd to run at half speed / power use)
1 external heater with 3 adjacent furnaces on the East, South, and West faces

Recreation:
Put 3 full stacks of items (logs, cobble, etc) in the furnaces. As the internal buffer of the external heater runs dry the 'heat' meter in the furnaces should falter, but as soon as this happens the eastern furnace deletes it's stack of items, and closes any open GUI a player had with it. Placing in any additional items deletes them as well as closing it's GUI. This persists until power is fully available again. The other two furnaces do not delete items, though they occasionally slow down a little.

Workaround: Buffer the power input so that the external heater never loses power. In this case a HV capacitor plopped down right on top did the trick fine. (I had just made a bunch and the player deserved it for their trouble).

Crashlog: pastebin

Versions & Modlist

IE 1.12-98
Forge 14.23.5.2854

When testing in a singleplayer minimal instance (only the mods from the above crashlog which start with the work "immersive"-- oh, except immersive cables, that one got disabled on accident) instead of a server I could not re-create this bug.