Probably not the t'stat area per se... the coolant is pressurized by the pump in the block and heads and intake all the way up to the t'stat even when the t'stat is closed.
But..... did you do flushes of the crankcase after the last time, to get all the water out? And primed the oil pump to push diesel through the system while slowly rotating the crank to get the diesel in all the passages. I have done so but it took 4 flushes to the get most of the water out, and I had a lot less coolant in that engine than you had. I just have to wonder if this is just residual coolant still in the engine that is now just coming out....???
Did the coolant level drop? (Of course, that would be impossible to tell if you still were getting air out of the cooling system.)
