I just tested it in game, it works but items are given only after node is fully mined (at last hit).
My config for reference
{
"Item gathered to get bonus": "stones",
"Permission": "extragatherbonuses.default",
"Maximal items that player can get by once": 0,
"Bonus list": [
{
"Shortname": "paper",
"Amount min": 1,
"Amount max": 5,
"Skin": 2783018053,
"Display name": "Coca Leaf",
"Chance": 100
}
]
}
None of my plugins are handling stack sizes so this issue is coming from somewhere else. I recommend using https://codefling.com/plugins/stack-modifier for custom items.