I'd like to think that Lamenters are so unlucky, that Marines Malevolent would actually be nice to them... only to make Lamenters look bad in front of other Chapters, where both have a history of already being distrusted across the board, and Lamenters actually sided with the Heretics at one point, and Malevolents are already being put into consideration of being ex-communicated.
Like by simply associating with the MM, it adds another black mark to their record, and the MM are making an active effort to tank the Lamenters' social credit for the joy of it.
Next thing you know, Marines Malevolent rain fire on the Lamenters, only for the chapter survive only by like 5% and ending utterly broken by the betrayal.
One day it's gonna get to a point where anyone not being an open asshole to them is going to start ringing the betrayal alarm bells.