Thanks for the suggestion, Volts.
As it turns out, I do have a MKVToolNix script for removing extra tags, but I’d stopped using it in an effort to adopt using video files in their raw form.
Running that script against these movies - and the ones from the other post - did in fact allow Plex to successfully transcode them, so your suspicion that all the different embedded subtitles exceeded a limit somewhere seems spot on.
I don’t suppose you know if there is there a different channel to use when filing a bug report? - or would I upload the .xml files here and hope that they get noticed and passed on to the dev team?
Thanks again for the help!
-Nathan