It seems there is an error with the managed speed function. For no apparent reason, it will change from Speed to MACH at low FL's.
As can be seen here, FL180 and it has switched over from Speed to MACH, while in the cruise.
The bus is indeed capable of flying transatlantic. Just make sure that in your realizm settings, you have unlimited fuel set, because the bus (specifically the A318) caps you off at around 24 kgs of fuel I believe. If you're going to use maximum fuel and passenger settings, just make sure that your flaps are set to 1 and that you apply TOGA thrust for takeoff, because that baby's gonna need a tremendous amount of power to takeoff.