WT CJ4 has a limit of FL450 (according to WT’s Simbrief profile).
I can’t reach that altitude, climb abruptly stops at 44600 ft, also airspeed drops instantly way below 100 kt IAS and aircraft sinks. Once sunken about 1000 ft climbing starts again, but only up to 44600 ft.
I first thought, there was something wrong with my setup of the WT CJ4, but then found this thread.