I had something like this happen once. Fortunately, I checked my playoff roster before the series started.
I don't think I can call this an error. It sounds like you had more than 25 players on your ML roster when the season added. The game has to have some way to pick 25 for the playoff roster. No matter how it does that, somebody isn't going to like it. So it's always going to require us checking that we've got the 25 players we want going into the playoffs.