I signed in to pay my AMF today. I have repetitive messages in my inbox stating a due date of April 30, but I observed in my portal that my calendar dates have shifted to an October-September window, and I'm not "due" until October 1st of this year.
Did something change pertaining to my account? I would have called to ask, but this is a more reliable method to ensure a response.
[edited to add: I went ahead and paid it, because the introductory screen showed the October-September calendar, but pressing forward to the payment window it reverted back to a May-April calendar.]
[I wish I had taken screencaps, but this is my attestation, anyway. --eg]
I'm really not sure why your date changed. It should be based on your first cert even if you add another cert. You can definitely send an email to membersupport@isc2.org to ask. It doesn't hurt to find out and since you've already paid, it's not urgent.