This was tested using paper 1.19.3-448 and paper 1.19.4-453 with the latest release of stackableitems on spigot.

I tested this using the .example items.yml file

StackableItems is working fine on 1.19.4 except for the new inventory changes that haven't been implemented yet. Were you by any chance in creative mode when you were testing this?

StackableItems is working fine on 1.19.4 except for the new inventory changes that haven't been implemented yet. Were you by any chance in creative mode when you were testing this?

Yep! I totally forgot that it only worked in survival.