CBR does not seem to work in mfh264enc and x264enc

Hi, I am trying to encode my monitor output to H264, mux it into MPEG-TS, and send it via multicast. I set rc-mode to cbr, so the bit-rate should be constant. However, when I receive this stream using VLC and check the statistics, the input bitrate varies. What am I doing wrong or what am I misunderstanding?

I am using the following pipeline:

gst-launch-1.0.exe d3d11screencapturesrc capture-api=wgc ! queue ! video/x-raw,framerate=25/1 ! videoconvert ! queue ! mfh264enc gop-size=25 bitrate=2048 rc-mode=cbr ! mpegtsmux name=mux ! udpsink host= port=11112 bind-address= multicast-iface= ttl-mc=10