As you can see in the screenshots if you grab a new jet engine and set bypass ratio and compression ratio to max you get impresive Isp values (around 20000s!!)
If then you change the engine to have an afterburner the values update to 6000s. Fair enough but after removing the afterburner it stays in 6000s.
I guess the 20000s is a wrong calculation that is updated after setting the afterburner on and off, but it may also be the case that the 6000s value after turning it off is wrong, I don't understand enough about jet engines.
@AndrewGarrison thanks