Don't know if it's the same on the FL2, but on some cars the speed sensor for CC, up to fifty is taken from the rear, over fifty it then uses a front sensor. This would explain why it works up to fifty, but just a stab in the dark!

Another possible cause could be a significant difference in tyre depth, or even one tyre being right down on pressure confusing the sensors.
As a matter of interest, if when the cruise control is working at 50 mph, what happens if you increase the set cruise speed via the + button on the steering wheel?