Common? Yes, more times than not for various reasons the relationship dissolves and both parties continue on living their separate lives. That is not to say they don't remain friends on some level though. I only know a handful of couples who have made it through the entire process of self discovery in the journey of a new gender role. Getting past the initial coming out is the easy part. Staying together in a healthy civil relationship for several years is a whole other story though.
I am not saying this to be negative and bring down the happy vibe. It is how I have seen things play out over and over again with those I know and myself as we continue to grow and discover who we really are as people.
It is usually the couples who have been together and have had a relationship for several years or longer before one of them comes out who make things last. If the relationship is strong it only increases the odds but doesn't guarantee anything.
There is so much that goes into being TG that I can't possibly fit all in this little box. When it comes to relationships though, it comes down to who wants to compromise the most to ensure the other person stays happy. Eventually those repressed ideas and feelings come out and that is where the relationships go to hell real quick.
Hopefully this helps.