By default, the issue is still reproducible as in the bug description. Setting the extensions.formautofill.heuristics.autofillSameOriginWithTop to True will solve the issue and if the the autofill is triggered from the card Number field, the brand, card Number and card Name fields are autofill accordingly. Card Number field is part of an iframe since card Name and type fields are in the main frame. about:autofill attached here.
Bug 1897463 Comment 3 Edit History
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By default, the issue is still reproducible as in the bug description. Setting the extensions.formautofill.heuristics.autofillSameOriginWithTop to True will solve the issue and if the the autofill is triggered from the card Number field, the brand, card Number and card Name fields are autofill accordingly. Card Number field is part of an iframe since card Name and type fields are in the main frame. Screen recording attached here. about:autofill attached in comment 4.