It's strange in my opinion. I know that some routes are new and experimental (e.g. PHL-IAH), but other routes are pretty much guaranteed to run routes (e.g. DEN-LAS, DEN-DCA).

I'd think Frontier would want to attain the additional revenue of purchased tickets for guaranteed to run itineries past October at this point, even if the full schedule isn't decided and Frontier is awaiting for load factor and revenue performance for the experimental routes.

Frontier likes to craft schedules partially on routes that operate seasonally, so summer performance might not be a factor for winter schedules anyways. For example, I'm sure TTN will be heavy on Florida with PBI and RSW possibly as daily, while ORD, DTW, and MSP are suspended.