Bookshelf uses the vanilla inventory (container) system for inventory interactions on fabric. This causes issues with mods like extended drawers which exclusively support the fabric transfer api (supporting the vanilla api would be hard due to the large item counts). I see that you already override the inventory lookup on forge to support inventory capabilities, the fabric implementation would likely be similar, although the transfer api doesn't provide slot numbers except for some storages, usually you only have access to an interator of views you can extract from. Insertion is only supported on the storage itself unless additional interfaces are implemented.

This was originally reported to me in a curseforge comment on extended drawers about botany pots not working with drawers.