I know this is months after your issue, but I am having the same issue on a live campaign that just caused some pretty significant problems for us. I had two nearly identical campaigns listening for form fill out and sending a confirmation email in response. One campaign had a "Date of Activity" constraint and one didn't, the one without the date constraint worked perfectly and the one with the constraint did NOT work (and I am 100% positive that the dates were right and that there were form fill-outs).
Either this is a buggy area of Marketo OR it's possible that when you set a "Date of Activity" it's a date field, but the date of the activity in the lead record would be a date time field and that's why it's not working.
If anyone can confirm this that would be helpful. But I know I'm definitely going to avoid using it from now on.