Instead of the normal 8, when you use a coal on a kiln it only smelts 4 items. I don't know if the same thing happens with other kinds of fuel.

Just found out that it's an incompatibility with Polymorph, and it also happens with Charm's kiln.

Can confirm that this is an incompatibility with Polymorph. Removing Polymorph restores the normal function of the kiln.

Environmental 1.16.5 - 1.0.0
Abnormals Core 1.16.5 - 3.1.1
polymorph-forge-1.16.5-0.24.jar

This is being tracked on the Polymorph github and seems to occur with other mods so please track it there