Looks like people were snatching up the available PN/PZ/IN/I as the day went on yesterday. Five seats were released on most/all Wednesday southbound and Monday/Thursday northbound flights until late October, and I saw that number progressively go down on certain dates I was monitoring.
Overnight, RM realized its "mistake" and pulled the plug on even P and Z inventory. Now most dates only have J and C, you'd even have to search around for D.
This is mostly standard UA practice when a route/flight gets added for booking. Just chronicling my observations in case it's helpful for future instances.