FLCH opens window with knots displayed
 

But never seems to open it with a mach number displayed on Boeing's(in my experience anyways). Is there a reason for this as mach can be subsequently selected.

I never noticed this, but it does make sense to me - flying current IAS will keep you reasonably within the envelope whether you are climbing or descending, maintaining Mach will result in a large change in IAS and a higher chance of hitting a limit.

On our fleet of NGs it opens up in whatever mode is commanded in CLB/CRZ/DESC pages.
e.g. If climbing KIAS before conversion to calculated Mach it opens in KIAS, descending from cruise level opens in Mach, unless the KIAS calculated for initial descent is equal to or lower than descent Mach number.
At lower (e.g. FL250) cruise levels, it always defers to KIAS

As described above in our FCOM 2:

737-800, opens in active mode, switches modes around FL260.