Why doesn't a form recognize info typed in using autofill?

[AC Question]

For example. I start to type my email address on a form in the box I'm supposed to and after typing the first letter the formfill info pops up so I tap on my email address and submit but the site says it's an invalid email address. So I type the email address in, ignoring the formfill and it works. Same email address, no typos. Why don't some forms recognize the email address when using the formfill? Same thing happens with address etc, anything that I used formfill for. Doesn't happen on all forms but quite often.