According to your Data Generation documentation, the ISlotData#operation function is supposed to take a String argument, but in reality it takes an AttributeModifier.Operation argument instead. This results in the data provider generating slot files that do not match the expected inputs when loaded as the vanilla "ADD_VALUE" operation is not the same as the "ADD" input that your code is expecting from the operation field.

Having said that, it does appear to still add the slot regardless (my assumption is that it's just ignoring the incorrect operation field, and using the default operation instead), but this is obviously not ideal.

Yes, you're correct on this. I'm not sure how this discrepancy occurred, probably just an oversight as that code hasn't been touched since its inception, but I'll correct this to take the proper String argument.