Hi, this sounds like it's a result of the Cure Critical Potions being bound to a quickbar slot and you're using saved quickbars. The complexities involved in saving items on the quickbar require that I set a variable on any item (or stack) you have in a saved quickbar slot.
When the variables on stacked items are not the same they will not stack. This is how I get around some storage related stacking issues as well.
I can't really think of another way to fix this off hand aside from what you've done with the oxen. The oxen remove any variables on the items. I understand the importance of being able to bind stacks to a quickbar slot though and I'll try to think of a way to remedy this.