Can't make multipart stacking work for three part movie

To begin, I have read all the threads on this topic and the support article here: https://support.plex.tv/articles/200264966-naming-multi-file-movies/

I am trying to stack this movie that is 7 hours long and appears in three parts.

I have named the files according the support article:

/Satantango (1994)
Satantango (1994) - pt1.mkv
Satantango (1994) - pt2.mkv
Satantango (1994) - pt3.mkv

Naming them this way has them appear as three separate posters in my library. When I then go and fix the match, they are bundled together, but plex thinks that they are versions of the same movie.

Is there a step I am missing. Should the files be named something else?

I have gotten multipart movies to work for me. I think your issue is the “part” nomenclature. I have had success with naming them as “xxx - part1.mkv”, etc. I don’t think that “ptx” is the correct format.

You need to omit the space characters before and after the hyphen.
This is different from the way series episodes are named, where spaces are recommended around the hyphens.

Followed your advice and it works.
/Satantango (1994)
Satantango (1994)-part1.mkv
Satantango (1994)-part2.mkv
Satantango (1994)-part3.mkv