I always put email in the default Username field so if I have multiple accounts on a site, I can see at a glance which one is which (and because that’s what most sites expect there). Then, for bookkeeping, I create a custom Username field to store the username.
The problem: for some weird reason, the extension will sometimes use that custom field instead of the default one. Why does it happen, and how do I stop it? Every time it happens I have to then manually copy-paste the actual email from the default field.
This happens because the autofill algorithm is designed to autofill custom field values into any form field for which the custom field’s label matches any of the HTML attributes id, name, placeholder, or aria-label that may have been specified for the <input> form element in the web page code.
Thus, for any web form in which there is an <input> field with id, name, placeholder, or aria-label attributes set to the value username, your custom field named Username will be autofilled into that <input> field (the field label matching is not case sensitive).
The simplest work-around would be to edit the custom field labels to be something other than Username (which is a fairly common input field identifier). For example, name it My Username, or Username for example.com, or Username-797468 (adding a random number).
A more complex work-around would be to add another custom field that is also named Username, but making this custom field a linked field (linked to the Username field that contains your email address), and placing that linked custom field before (i.e., above) the custom field that has your non-email username.