I spotted this issue the day after an update to 3.2.2
Tizen : 2.4.0
Plex for Samsung : 3.2.2
Updated : 2018-02-08
PLEX Server : Official Synology version 22.214.171.12423. "An update is available", however this would be a manual install, not the Synology package (which in due course, I would expect to become available). The release notes do not refer to aspect ratio explicitly or implicitly.
I have seen the problem only on MPEG2 files at this time. (I have not checked everything in my library).
Most files in my library are mpeg2 or mpeg4 files (unmodified DTV broadcast streams) encapsulated as mkv.
HD (1080) material plays fine, full screen.
MPEG4 material that isn't 1080 is also playing fine : eg 720p, 480p material
MPEG2 576 line material is always squashed horizontally. 720 pixels are squashed into a 4:3 frame; 544 pixels (SATV sources) are squashed even more.
I have tested a small number of mpeg(2) files (as opposed to mkv files) with the same results.
Playing videos through a web browser continues to work correctly.