Dispatch always sets cruising altitude at FL200 or FL210 regardless the length of flight? Ceiling for these airframes is 35,000.
The B463 is one of the legacy SimBrief profiles and isn’t very accurate.
For now it might even be better to use the RJ100 (RJ1H) profile instead, or create a custom airframe based on the RJ1H and modify the name, weights, etc to match the B463 if you prefer.
Best regards,