I had a trip in my cart and removed it to see if we could shorten it by one day, then immediately went to check availability - but now both the original dates and the one day shorter timeline are unavailable for the room.
Does anyone know if the site “holds” reservations after they have been in the cart for a certain amount of time, or do I just have very bad luck? Thanks!
Probably a glitch but could be real depending on your dates and the resort in question.
It was available again this morning, so whether an intentional hold or some kind of glitch, anyone else with this issue can hold on to a glimmer of hope 
I didn’t answer the hold part. The answer is no. 
Yes I’ve had this happen, both on Disney and some other reservation systems and online stores too.
I think the reservation/dates are being held for you while in your cart, but when removed, there can be a delay before they become available again. so sort of a glitch I guess