Look in the game folder. Find and open in a text editor this file:assets\data\Items.gd
Search for spearadv
In the description of the spearadv find these two lines:
tags = ['recipe'],
slots = ['rhand','lhand','no_stack','enchantable']
Change them to:
tags = ['recipe','no_stack','enchantable'],
slots = ['rhand','lhand']
This should fix issues with new great spears but likely not ones you already have.