Just in case anyone has a similar experience with seats on Spirit, for another data point, the exact same thing happened on our flight back from FLL-ACY. Tons of seats (including middles) were initially showing as open and offered for purchase, and then the day before the flight, basically all of the middle seats were suddenly unavailable. When we went to check-in, three of us were auto-assigned a window seat, an aisle seat, and the other aisle seat across the aisle. No one ended up in the middle seat that was skipped/blocked, and it seemed to be exactly the same for most rows around us. It was actually pretty nice and almost felt "spacious" for Spirit. Almost every middle seat around us was empty, but windows/aisles were all filled. There were a few people in middle seats, but I imagine those were people who proactively paid for those seats further in advance when they were being offered for a fee.

My best guess is that on flights that aren't full, Spirit's algorithm blocks out middles, tries to assign only windows and aisles, and if more seats end up being needed/tickets sold, there would be the option to pull some of those middles back into use. That may have happened on our flight to FLL a couple days before, where we were auto-assigned a full row of 3 on a fuller flight, despite middles having shown up as blocked before check-in.