An update to the various Plex TV apps that had this issue was released yesterday
Please see the various Plex fpr Smart TV forum announcements
Plex for Samsung
Version 5.77.1
Fixed an issue where the last line of subtitles don’t get shown
and similarly
Plex for WebOS 3.0+ (LG Televisions)
Version 5.77.1
and
Plex HTPC for Mac, Windows, and Linux - Version 1.56.1
Plex for VIDAA (Hisense television) - Version 5.77.1
Plex for VIZIO Smartcast - Version 5.77.1