The Bug

Advancement trigger inventory_changed can't consider multiple slots, so if you have an advancement where you must collect 5 feathers, but you have 3 feathers in one inventory slot and 2 in another, it won't trigger. This also makes it impossible to trigger advancements where you must collect more items than the stack size will allow you to fit into 1 slot.

How to Reproduce

Download Paint's Bug Tester Datapack (attached), there are many advancements on there, but the only one for this bug is "17 eggs", where you must collect 17 eggs, but it's impossible to trigger.