On the Orders page, it’s confusing to see how a partially filled order is categorized. Alpaca Orders page calls it “canceled”, since there are only 3 options and “partially filled” is not one of them. This led to confusion, making it appear as though the shares that were filled (yet marked as cancelled) were later sold, making it appear as though it created a short position.
The reply I got from support was that the Activities page gives more clarity. Indeed, it does, but nonetheless, the Orders page is confusing and misleading.
Requesting this bug/behavior to be changed to be more intuitive.