If you need any help troubleshooting this issue I started a now locked thread and was able to get to the root cause with the help of @anon18523487 and @ChuckPa. They were each on the cusp of filing a bug report but unfortunately I don’t think it ever happened. Here is the thread:
Plex Incorrectly Estimating Required Bandwidth - Forcing a Video Transcode
The tl;dr was that MKVToolNix (and possibly FFMPEG) changed how they handle audio bitrate metadata for MKV’s. Older versions of MKVToolNix used:
Metadata:
BPS: 640000
BPS-eng: 640000
However newer versions simply use:
Metadata:
BPS-eng: 640000
PMS is unable to parse the audio bitrate from the new metadata format. If you attempt to play the file before a deep analysis occurs (or simply have deep analysis disabled) it can cause lots of wonky behavior on a number of clients since Plex “streaming brain” assumes an astronomical bandwidth requirement due to the blank field
You should be able to recreate an affected file yourself by using the following command on any old AVI/XVID file with a modern version of FFMPEG:
ffmpeg -i movie.avi -c:v libx264 -c:a eac3 movie-h264-eac3.mkv
For anyone experiencing similar issues, me and @Volts have discussed the issue further with some possible workarounds here: