Yes, Ancestry and RM treat family events, like marriage, differently. This has been discussed in other threads here and on facebook (for example - TreeShare marriage records on add new individual ). I try to import the people first, including the spouse and then the events, so it’s always a multistep process.
Treeshare will show you where you are missing Marriage events and you could manually download them one at a time using treeshare. (There may be the same issue with divorce as well.) That’s probably the best method if RM is your master file. If ancestry is your master, then a fresh download to a new RM file would be simplest, Going forward, you might want to refresh treeshare after downloading data to verify that all data came across. I view/edit every ancestry citation that I import via treeshare to transcribe citation details and fix source data that ancestry doesn’t send automatically. This also serves as a validation.