Migrated / consolidated from old repo:

chimericdream/hopper-xtreme#2
chimericdream/hopper-xtreme#5

Final comment with likely root cause and fix

Someone left a note on my cf page that hopper doesn't work with SS. Just briefly checked the code and I don't believe that I see any IItemHandler capability implementation anywhere, which was SS and I believe anything else in neoforge ecosystem uses also Neoforge wraps all vanilla inventories with this capability so that can be universally used to access any inventory and hence is the way to go when working with items. https://docs.neoforged.net/docs/datastorage/capabilities