Plex reads the ‘Peaks and Valleys’.
There are 23Mbps Peaks in that encode - and Plex knows what those are, so it’ll know if a file will fit into the hole you want it plugged in to and if not - transcode.
Either change the way you encode, or acquire, or give Plex plenty of headroom.
Regarding this - a few users are digging into the details here - it may be of interest:
