CBR is not really CBR
CBR is not real CBR.
With every CBR settings in media encoder, final export always has the lower data rate rather than the chosen data rate.
Antoine (Autokroma.com) commented
This is normal since 50 Mbps is a pretty high bitrate for 1080p.
You could also try with AfterCodecs it has another H264 encoder https://autokroma.com/AfterCodecs/