Ok then will give you some info in case it helps:
This plugin adds an extra slot which is not visible in the UI when the headlight is on (a 9th slot after backpack slot), it does not use the visible ones.
There is harmony patches and a hook that makes the game ignore this miner's hat when checking for headwear.
Lets say the harmony patches fail, the vanilla behavior will not that it would stop you from wearing the helmet, it will pop out the miner's hat to your main inventory (which doesn't seem to be happening in your case, and should also apply to hazmat suit)
My suspicion is maybe a plugin is using CanWearItem hook and looping into all slots (including one with headlight in it), and preventing if it finds another headwear?
Also ofcourse i am assuming you're using version 1.0.9.