I just have this feeling that the crew became incapacitated for one reason or another and couldn't eject.
You know, of note, if those flightaware altitudes are correct and they were stable at 8000' initially, there is no pressure differential in the cockpit at 8000', the altitude where the pressurization system kicks in during a continued climb. There are various filters in the pressurization system. Are they active before a cockpit pressure differential is available?
I recall, even in the always unpressurized T-37 jet trainer smelling fumes at times (no not that) during various extreme maneuvering gyrations, momentary tail slide for example. The T-6 stacks are about in the same relative vertical position as the T-34, just a little further forward on the long nose, tail slide not required.