If it’s not possible to determine future fatigue is it impossible to predict future illness? Say you’re on reserve and receive the appropriate notification sometimes well over 12 hours in advance and after notification you or someone that you can use qualified sick time to assist later becomes ill. It’s inconvenient for Frontier but it should not be punishable. Especially for pilots that must sign fitness for duty.
However, I can see that if a pilot were to be on short call and receive notification of a 2 hour callout and the pilot then advised CS that they are calling out sick is more difficult. There may still be valid situations. Kids and family are always a bit of a wildcard.