I had the same issue last night on the stock A320 but with a third party livery. I initially programmed the flight in the FMS, going direct to KMSP with no arrival info entered (waiting until I get runway assignment from ATC for that). Once assigned a runway, I updated the flight plan in the FMS without issue. I then messed around with trying other arrival routes, which took the plane off the original arrivals course and it didn’t seem to want to correct when entering the original back into the FMS. I set a direct to for the transition waypoint to force it back on route and the flight plan screen goes to UNTITLED. I did notice that once I reached said waypoint, the flight plan screen goes back to normal and showing the rest of the legs, as well as following them correctly.
I found this thread via a Google search, but this sounds like a long standing bug that hasn’t got much official reporting. This bug report from 2 years ago auto-locked due to lack of replies, but it’s the same issue.
I did see a lot of complaints on other forums about the untitled screen as well during my search.