Hi,
I acknowledge this is a bug. However I'm afraid it is a bug that will be very difficult to fix for a number of reasons and is not likely to get fixed in the near future. Part of the problem is that the Recaptcha control is a validator but it does not support the ValidationGroup property that would be needed to isolate different validation scenarios on the same page.
So my recommendations is don't put a sign in module on the same page with Form Wizard or any other feature that uses a captcha.
Best,
Joe