https://twitter.com/InclementDab/status/1656785863114735618
based on this report we require further information/feedback consolidation about this.
The introduction of the new config mid classes has caused issues for the mods, using previous inheritance (mostly missing inventoryslot parameter) resulting in the vanilla (overriden) clothes not possible to equip anymore.
We will be reverting the config side of these changes for the next 1.21 experimental release, meaning no necessary changes will need to be made on mods side.