The HTML input element has a type attribute and a name attribute. Those values and the values Chromium supports for autofill determine what gets stored and autofilled. You can click "load more" at https://source.chromium.org/chromium/chromium/src/+/main:components/autofill/ for details.
I would right-click the text field and choose "inspect element" to see what type and name the username and email address fields have to see if they're the same. If so, that might explain it.
I'd also goto the URL opera://settings/addresses and review the autofill data to be sure your email address isn't set to autofill for username fields.
I'd check at opera://settings/passwords and see if the username for the login data is your email address. If so, and it's not supposed to be, you can delete the entry and add in manually correctly.
When typing in the username field, if your email address pops up, you can select it with the down arror and hit shift + del to get rid of it.
While on the page, you can hit ctrl + shift + i to open the developer tools. Then, you can goto the Application tab, select "Cookies -> the site" on the left-hand side and check out the value of the cookies for the site on the right. Your email address might be stored in a cookie for that username field where the site itself reads that cookie and fills it in. You could also just clear your cookies for the site to rule that out.