With only Fabric API, Trinkets, Inmis, and Trinket Shulker Boxes enabled, Shulker Boxes are unable to be equipped in the back Trinkets slot. Disabling both the "enableTrinketCompatibility" and "trinketRendering" options in Inmis' config did not fix the issue, and the only solution which worked was disabling Inmis. Any Shulker Boxes which were in the back Trinkets slot before Inmis is enabled are able to stay in the back slot, but upon being removed from the slot they cannot be placed back in the slot.
EDIT: In case this information is needed, I am using Inmis version 2.5.2, Fabric API version 0.48.0, Trinkets version 3.3.0, and Trinket Shulker Boxes version 2.0.1