score:4
I travel regularly with Flixbus, and there'll be no issues. A lot of the time I don't even buy a ticket for the whole journey I'm actually making, and I got nervous once, when it got sold out on the section I hadn't paid for, that they might check everyone's tickets and throw me off. Somehow, however, it was sorted out without any issues.
Obviously, since you (presumably) have a valid ticket for the whole trip, there are no grounds whatsoever for any problems.
You don't get a specific seat either. So really, you're fine.