Forerunner 735xt and phantom DF in activity... how to reset?

Hi,

I know this is more a support issue than a forum one, but bug on Garmin support page means no chat is available.

So here is my issue, bonus points for anybody who knows the resolution:

My 735xt insists I am using 3/3 ConnectIQ fields so cannot add any more to my DataScreens... but watch is _WRONG_

I have checked all screens, I have even reset all fields in all screens, I have turned all screens on and off again, I have synched.

Watch still thinks (and stores data from) a phantom DataField that I cannot remove from the activity.

Any ideas how I reset?

Thanks,
G