Environment

MC: 1.15.2 (or 1.12.2)
Forge 31.1.0 (or 14.23.5.2847)
AutoRegLib: 1.5-40 (or 1.3-32)
Quark: r2.1-245 (or r1.6-178)
Iron Shulkerboxes: 1.15.2-4.0.1 (called ironchest for 1.12.2)

Bug

Right clicking items into a shulker box (in your inventory not placed on the ground) with more than 27 inventory slots discards all items that are not within the first 27 inventory slots

How to reproduce

Fire up a minecraft in 1.15 (or 1.12) with the mods listed above.
Get yourself a larger shulkerbox from Iron Shulkerboxes (iron or copper shulkerboxs are enough but any boxwith more than 27 slots works) place the shulker on the gorund and fill it completely with junk.
Remove all items from the first slot.
Break shuklerbox.
Right click something into the box. This voids the items.
Place shulker on ground and look for yourself.

Small feature request

If possible could you also add support for right clicking items past the 27th slot.
So if the first 27 slots are full but there is still space (after the 27th slot) you should be able to add more items by right clicking the box in your inventory.

Also please consider to fix it in 1.12.2 additionally if you still support 1.12