I often hear Novus Ordo Catholics accuse Latin Mass Catholics of being "Sedes" and rejecting them as Heretics.
Of the various Latin Mass Catholic orders that I know about: FSSP, SSPX, Institute of Christ the King and SSPV none are officially Sedevantist. The SSPV takes a neutral stand.
There is a group lead by Bishop Sanborn that is Sedevacantist.
Here is a short summary of their argument.
Now, imho, the various Catholics don't differ on matters of faith, just on matters of fact. When a Pope does things that can be viewed as heresy, it is sure to cause scandal and doubt.