I just joined nanog to allow me to respond to a thread that Simon posted in March. .
(Not sure if this is how to respond)

We have the exact same problem with Aruba Access points and with multiple MacBooks and a iMac.
Where the device will spoof the default gateway and the effect is that vlan is not usable.

I also have raised a case with Apple but so far no luck.

What is the status of your issue?  Any luck working out exactly what the cause is?

Regards,
Mike