garmin

1] Above the Change Email link. This is not a form field, so it won't be prefilled

2] On the Account Details [read-only] page, after the user presses Change Email. This is still not a form field, so it won't be prefilled. There is an Edit button on this page.

3] On the Account Details edit page, after hte user presses Edit. This is a disabled form field, so hopefully it would not be prefilled. There is a Change button next to this field, and when you press it, you're taken to a form which finally allows you to enter a new email address [without displaying the old one]

Even if 3. was prefilled, you'd think that OP would notice 1 and 2 were different from 3.

I think it's interesting that you are likely aware of all the different reasons [I listed above] that it's implausible that this apparent problem is wholly caused by the browser autofilling form fields, yet that's your preferred explanation for all of it.

It's also interesting that you initially seemed to believe that OP was reporting that they saw the login credentials being prefilled in the login form, which caused you to adopt this explanation.

"If the OP sees an email address prefilled in the sign-in form"

When it was suggested that the OP did not see any such thing, you continued to double-down on this explanation as the cause of everything that OP is seeing.

I do think that a video demonstrating the problem would the best way to convince Garmin that there is a problem. Too bad there isn't really an easy way to post such a video here, while redacting personal information.