Demonstration: https://youtu.be/T7VtmETvMK0

To my knowledge this achievable in the latest 1.16.[3/4/5] verrsion.

Chute: Transfers up to 16 non-stackables simultaneously when chute is placed below a chest that contains the items.
Output of chute into world stacks given items.

Brass Funnel: Grouped non-stackables (by specifying output stack-size) remain stacked when exiting conveyor into world (stacking up to 64 Non/limited stackables).

Aside from allowing stacked armour, sandpaper and filled buckets: mods that add portable storages that can be interacted via inventory cause item duplication - Quark inventory shulker input, Druidcraft travelpack, Toolbelt toolbelt. Stacked baubles from Curios will sometimes stack bauble effects (dependant on bauble in question).

Stacking of non-stackables during item transfer makes perfect sense: I'd assume most reliable fix (to prevent other mods from maintaining issue), would be to assign a temporary tag to all conveyor/chute transported items that forces unstacking once removed from conveyor/chute.

sidenote: to obtain stacks up to 64, input to Brass Funnel must be of a capacity greater than 64 non stackables, or to pre-stack the non stackables to smaller quantities to fit into Brass Funnel input.