I totally agree, this is a very frustrating trait - to convert to a checklist upon the return key. Just turn that off and lets us insert the next checkbox we want. Why do you "have to" convert to a checklist? I can do that by selecting a checklist from the start.
I keep a shopping list of all the typical items I buy at Costco and then I check the ones I want to buy this trip. I don't want the strike-out as it makes it seem that I have already purchased that off my list, not that it is pending. Thus I use checkboxes and check the items I want that trip, then on my phone, I uncheck the box when it is in my cart.
Just disable the "conversion" behavior, easiest code change ever.