One evening, while at BFI, a couple of Max test aircraft taxied out. One was painted up in Fly Dubai colors and the other was still in primer although the rudder seemed to indicate that it will be a United jet when delivered. They both taxied out and then held on the taxiway. The primer jet was second in line and held for ages. Eventually, it took the intersection on to the runway and backtracked to the Boeing ramp. No idea what the issue was but it clearly wasn’t ready to fly. Their colleagues took off a short while before in their, apparently serviceable, jet.