UUIDs are how Minecraft determines the IDs of attributes, if they have the same ID they will conflict with each other and only use the first attribute, this is likely how #1 happened.

I will do this soon ^^

The solution is to add a UUID option, so players don't get conflicting attributes

A simple fix would be to add the item name into the UUID generation, no extra config needed.

EntityAttribute attribute = generic_attribute.getAttribute();
Collection<EntityAttributeModifier> newModifiers = new ArrayList<>();
var identifierString = stack.toString() + generic_attribute.getString();

newModifiers.add(
        new EntityAttributeModifier(
                UUID.nameUUIDFromBytes(identifierString.getBytes()),
                "Custom Item Attributes Modifier",
                generic_attribute.getDouble(),
                generic_attribute.getOperation()
        ));

hm yeah that's a good idea! will implement this on the next version :)

Fixed on 2.0.0, please read full changelog as the config moved locations and the format also changed.