More specifically, if the Upgraded Shulker box holds more than 27 stacks of items and I either pick up the box from inventory and right-click to deposit a stack of items into my inventory -- or if I pick up something and then right-click on the shulker box to add it to the shulker box inventory, all items after the first 27 stacks disappear.

This is quite frustrating when I have diamond shulker box, possibly with hopper upgrade, and it is 3/4 or more full and trying to quickly add a stack of personal items to my inventory, most of the shulker box contents go away. That took LOTS of mining!!

Anyway, I'm playing on Minecraft 1.19.2 via the Curse Forge Overwolf client. The profile is based upon "Better MC [FABRIC] 1.19.2", though I've added a bunch of mods (like yours) and disabled a few that come with the modpack.