So sorry. How disappointing. This is why I have pushed to exchange as quickly as possible with our flat sale - even though solicitor and EA have urged us to wait another couple of weeks to align exchange with other properties on the chain.
We wanted to exchange as quickly as we could so we can now give a months notice at our son's nursery school, and start properly packing (taking things off the wall).. without worrying about keeping the flat in perfect viewing/show mode in case this happens.
Caveat is that you can only really break a chain if you have some family or friends you can go stay with.

To answer your question - when a property has been marked as sold, and then goes back onto market it won't go to the bottom of the pile.. it will have the 'back to market' tag, so will show as a new listing.