This is caused by redirect tainting in Firefox (tested by disabling `network.http.origin.redirectTainted`). I guess we could extend bug 1800990 to also cover redirects caused by `upgrade-insecure-requests`.
Bug 1800990 Comment 2 Edit History
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This is caused by redirect tainting in Firefox (tested by disabling `network.http.origin.redirectTainted`). I guess we could extend bug 1781772 to also cover redirects caused by `upgrade-insecure-requests`.