The auto unstacking does not work if the item is transfered to the player inventory through mining. This happens for:
A) mining a belt with a stacked item on it
B) stacked items that are collected by mining an entity with an inventory
C) mine an stacked item on the ground (instead of collecting it with the f key)

I think A and B are cases should be covered by https://lua-api.factorio.com/latest/events.html#on_player_mined_entity
And C by https://lua-api.factorio.com/latest/events.html#on_player_mined_item
or

I made a pull request :)

Thanks a bunch! Sorry for the delay, I should be able to get it tested and merged today.