KER 1.1.8.3 on KSP 1.11.1
Suicide burn values for some high speed, low altitude suborbital flights are impossible - occurring after impact. The suicide burn countdown is longer than the impact countdown. Also the suicide burn altitude is lower than the impact altitude. The impact parameters seem correct. The suicide values sometimes toggle between possible (above ground and before impact) and impossible (below ground/ after impact).

Screenshot on Mun landing approach (west-bound) with engines off. Launched near west end of equatorial Canyons. Stayed suborbital.

Happens to me while using Dawn or Twitch Engines.