Some NBT tags like SkullOwner is not being picked up on. After output the assigned item only uses NAME nbt tag and not skullowner

Example:

    "data": {
        "display": {
            "Name": "nametaginput",
            "SkullOwner": "skulltaginput" This is not being used at all! item returns with 1 nbt tag.
        }

This happens for this tag since Minecraft directly parses and writes the display object. Additional properties get discarded.
You can attach own properties to most other objects but not that one ;)

By the way if you want to give a skull a player nbt data the format looks like this:

		"data": {
			"display": {
				"Name": "nametaginput"
			},
			"SkullOwner": "Siphalor"
		}