When you put shulker a in a chest and a hopper below, then inside the hopper, the shulkers stack to 64 even though i’ve set it to stack to 16. Or you can recreate it by feeding a hopper full of shulkers into a container.

I think you're referring to Carpet Extra's emptyShulkerBoxStackAlways, so I'll move the issue there.

Can this also be added for 1.19.4? that’s where I experienced the issue?

You could backport the simple change here to the 1.19.4 branch and that should do it, Carpet Extra doesn't usually publish backports (lmk if you need any help)