Looking for advice on booking a one way trip...

Traveling MCI-RSW, one stop in STL and the WGA fare is sold out...anytime fare is 586.00. If I look at booking the two legs separately...MCI-STL and STL-RSW, both have WGA fares still available for a total of 405.00. Which in my opinion seems strange that WGA is not available on the whole trip...

If I book the legs separately I realize I can't check a bag but is there a possibility one of the flights will be auto canceled ahead of time.

Thanks