Although it's possible that there could be aircraft swapping back and forth between Midwest and Frontier, I think this is perhaps more likely in this circumstance:
Except Tuesday/Saturday
06:20 DEN dep
11:45 MCO arr
12:20 MCO dep
14:15 OKC arr
14:50 OKC dep
18:30 MCO arr
19:05 MCO dep
21:17 DEN arr
Tuesday/Saturday only
06:15 DEN dep
11:30 TPA arr
12:05 TPA dep
13:50 OKC arr
14:25 OKC dep
17:55 TPA arr
18:30 TPA dep
20:35 DEN arr
Although the DEN flights are not timed for connections, there's plenty of local DEN demand for an E190 to Florida. The first nonstop of the day DEN-MCO and DEN-TPA both arrive after 4:00pm.
The other thing which makes me think this might be the direction is that there hasn't been any sort of ancillary new route press release. That might still be coming, but the added 5x/week DEN-MCO and 2x/week DEN-TPA are not a new route nor a big expansion, just another flight showing up for sale certain days of the week.
Not saying this is 100% certain, nor that they will never have instances with aircraft flowing between F9 and YX daily, but I suspect this could be the answer in this particular case.