Are you sure that it is jammed?
My wr450f has a similar problem: If the engine comes to a stop at just before top dead center the compression is so great that I can't kick it over. The electric starter can't crank it either. Both the kick starter and the electric starter won't crank it.
If I stand on the kickstarter it will slowly (very slowly) crank over as the compression leaks by the rings.
When this happens I just drop it into gear and back the bike up a bit, just enough to give me a running start on getting over TDC. When I am lining up for the start of a dead engine race I will do the same thing to make sure that I am not just before TDC.
My daughter has a CRF250R and has never experienced this situation.
Rod