I have just come across a strange.... glitch is the best way I can put it having ripped The Mechanic (1972) - https://www.blu-ray.com/movies/The-Mech ... 311952/.
When I started watching it in Plex, it was the correct aspect ratio but in a small square in the centre of the TV screen. I tooled around with TV display/aspect ratio settings and the same things in Plex but couldn't it to display correctly, despite Playback in VLC looking right.
I ran MediaInfo on the file and lo and behold, the Display Aspect Ratio was set to 1.0, with the Original Display Aspect Ratio showing 16:9. I fired up MKVToolNix and could see that the Display width/height was set to 1080 x 1080. I fixed that, remuxed it and the new file displays correctly in Plex.
I guess this is an authoring error, or is it a bug in MakeMKV?