The same limitation is in the CL300/350 too, and to common to all Bombarider Collins FMS equipped aircraft. The reason for the limitation is that if for some reason the FMS looses the speed values entered into the PERF VNAV CLIMB page, the climb Mach speed reverts to M.50, which would be very bad if you are climbing in VFLC and the FMS ACTIVE SPEED on the PERF page in a ACTIVE <mach> speed, i.e., the speed bug and VFLC speed reference is pink. The airplane would pitch up and try to track M.50. At FL330, that would get exciting!
You can climb in VFLC with and IAS value in the PERF ACTIVE <speed> active, i.e., a pink speed bug set to IAS, but when it transitions to Mach, you need to change to FLC, or you can move the speed bug which will change the target speed from the FMS ACTIVE speed to a FCP speed, i.e., CYAN/blue bug. Simply put, when in FLC or VFLC, you want the target speed bug showing cyan/blue.
Rich