To reproduce the issue install the latest create mod for 1.16 and the FastFurnace mod with its dependency Placebo, together with Polymorph

Then after placing a powered fan with lava in front of it and trying to smelt cobblestone, the game will crash.

I already searched both mods repos, but this seems not yet to be reported and I am not sure which mod is
actually causing the problem here. Removing FastFurnace or Polymorph fixes the issue, but having them both together seems to cause problems. Feel free to close the issue if one of the two mods are the culprit.

Here is the gist of the crash log: https://gist.github.com/Paspartout/dc02231c4c984e8503c45b9e41b90560

I can confirm this doesn't only happen in that niche scenario, but in what seems to be most scenarios that involves preforming a furnace recipe with a powered fan, as I tried a similar thing within the Valhelsia 3 modpack, which included the FastFurnace mod and Polymorph mod.

Instead of putting lava in front of the fan and smelting cobblestone, I decided to instead bake bread from dough by pulling in smoke from a camp fire. The game then proceed to freeze and crash, hence the snipped screenshot here:

And the crash log as well: https://paste.ee/p/vgzCH

Are you sure that's the right crash log? It shows some client gui problems while this problem occurs on servers too.

I got the crash log from MultiMC's other logs window, so while I don't know for certain how useful my log is to this problem, I can still confirm that the glitch happens on my end.

Since polymorph recently implemented furnace support and they are the topmost entry in the crashlog, they most likely are the culprit I think: illusivesoulworks/polymorph#8

This is unrelated to create, we are simply calling a vanilla method to get the smelting result. It's an incompatibility with fast furnace and polymorph as fast furnace is removing fields for some stupid reason.

This was fixed in a polymorph update: illusivesoulworks/polymorph#45