Since updating to 1.4 around 2 weeks ago, I noticed my colonists were not smelting helmets the way they used to. The list of possible ingredients for the bill Smelt Apparel no longer lists simple or flak helmets, at all. Marine and Recon helmets are still smeltable. Simple and flak helmets are still listed as possible ingredients under Armor>Helmets in the bill Destroy Apparel at the electric smelter, too.
Through process of elimination, I narrowed the culprit down to Rimefeller. My favorite mod. Removing Rimefeller restores the helmets to the bill ingredients list, but half of my base is dedicated to Rimefeller content, so I can't just delete it. I wasn't aware that Rimefeller affected smelting, armor, or helmet properties.
I haven't figured out why Rimefeller does this, surely it's an incompatibility with another mod. I don't know of a good way to check for mod incompatibilities. I use RimPy for sorting and also Fluffy's Mod Manager.
I'm not very experienced with the debug log. I don't really know how to read it, but my debug splash screen doesn't seem to offer any solutions. But I can try to provide whatever debug logs you want.