It's Chrome fault to use push for a new standardized header without properly checking its impact: the issue has been raised during the beta phase, but they underestimated it and did not rollback.

I believe it is known that some old reverse-proxies used this header before a standard solution emerged (X-forwarded-proto), Wordpress probably had to deal with this surprising behavior at some point.

Also, by naming a header "HTTPS", one could expect collision with a non-standard behavior: this is very common with HTTP/1.1.