If
{plex.name} is
MovieTitle.eng then
M:/Movies-1/{plex.name}/{plex.name} is
M:/Movies-1/MovieTitle.eng/MovieTitle.eng. You'll want stick to the
{plex} format as-is
(and not re-arranged bits and pieces) and everything will work out of the box.

If you want to customize your destination format, then you'll want to thoroughly test your custom code in the GUI on both movie files and subtitle files, and integrate it into your command-line call only after you've checked all the corner cases.
https://www.filebot.net/naming.html
e.g. use
{plex} format file name part:
e.g. use
{plex} format path without the TV Series / Movie folder part:

Do not blindly copy and paste my code into your command. Use the FileBot Desktop application and the Format Editor first to try it for yourself on a representative set of sample files, i.e. both video files and subtitle files in your case. In general, try to avoid copying code you don't yet understand and have not yet tested yourself in the Format Editor for all your test cases, because others will cut corners that you maybe can't, depending on your use case.