When in the upper atmosphere (Kerbin 50k+ usually), where the Thermal Turbojet would flame-out on ATM, sometimes other fuels (LH2 and LF) will also flame-out. Sometimes save/reload will cause the flame-out to end. Almost always (but not always), leaving the atmosphere will end the flame-out condition.

KSPIE 1.19.2

Same issue here, although it only seems to do this when using LH2. When I use Helium it works just fine, bu (but ISp of H-4 isn't very good compared to H2)

Thermal turbojet is basically unusable for SSTO because of this.

I note the thermal rocket nozzle doesn't have these issues. You can switch from "Atmosphere/L2" to "L2" after a flameout and it works just fine.

I'm guessing the "Atmosphere/L2" was supposed to be an auto-switch capability but it's broken.

Note that flameout with more than one engine leads to a very nice flat spin... so you want to avoid that.