
It could also just be an internalized fear that is showing itself this way. For instance- not saying this is the case- but if your ex was a horrible person who left you, the dreams featuring him might have more to do with a fear that your new bf would do the same. It could have more to do with emotions than specifically the fact that it’s manifesting as your ex.
In my experience, the first thing that comes to your mind when you think “boyfriend” is going to be whichever person stood out the most or made the biggest impact. So when you go from not saying that word for a while to calling someone new your “boyfriend,” the feelings and thoughts associated with the person who made the biggest impact will also appear. In my own relationship, it took me over six months to stop accidentally calling my boyfriend the name of my ex of two years.