I just got this response from Vista:
Hi Mike,
As said in previous email, this is a Mach3 bug.
From the pendant side, I can send you a modified plugin that disable the function of feedhold, so the start/hold button only has function of start. This will avoid to put the machine in hold position when press the button.
You can assign any one of the F key to feedhold Funtime.
Thanks,
Lee