Adobe Media Encoder (CC2019) isn't encoding at close to the specified bitrate with CBR

[A.I.1]

I've been asked to render a video at 20 Mbps.

When I render the video with AME CC2019 and specify a constant bitrate of 20 Mbps the output actually averages 6.2 Mbps according to MediaInfo.

When I specify a constant birate of 30 Mbps the output is actually 8.8 Mbps according to MediaInfo.

I can understand why using a VBR target birate of 20 Mbps could give less than the specified amount (and it does) due to the video having simple to encode sections, but surely specifying CBR should render a video as close to the specified bitrate as possible, no matter how easy or difficult the content is to compress.

When I encode CBR I use profile main, level 4.1, H264, video res 1080x1920 (portrait format), 29.97 fps.

How can I make it render the video as h264 at approx 20 Mbps avg?

Would I need to render it in some other format and then use handbrake or something to make the video for delivery 20 Mbps avg?