Both the old and new players can not recognize these are being english subs…
Plex Documentation → Your Media → Adding Local Subtitles to Your Media
That is not a supported naming format.
Plex uses ISO-639 codes for external subtitles.
If desired, you can embed the subtitles into the media file using MKVToolNix or similar tools. You can then give them a track name, which Plex will display in most (but not all) clients.
Umh this is a shame, but ok understood.
