TYPO3 Core

Agreed. Just stumbled over this as well :-)

Workaround for the moment in .htaccess:
RewriteCond %{REMOTE_ADDR} "!remoteproxyip"
RewriteRule ^typo3(.*) https://remoteproxydomain/remoteproxypath/typo3$1 [R,L]

Background of this issue:
TYPO3 checks for localconf-settings reverseProxyIP and reverseProxySSL. If the request comes in via the proxy (and reverseProxyHeaderMultiValue is set to first or last) the hostname is correctly fetched from HTTP_X_FORWARDED_HOST (see t3lib/class.t3lib_div.php). But to set the hostname which is used for redirecting from HTTP to HTTPS we might need a new setting like reverseProxyHostSSL (note that reverseProxyPrefixSSL already exists) or maybe "lockSSLHost" (since we already have $TYPO3_CONF_VARS['BE']['lockSSLPort']). I think going the reverseProxy...-way might be best since we can then also use the prefix-variable etc.

Currently in typo3/init.php the current servername is fetched and simply prefixed with "https://" in front.

dupe

Closed as a dupe of #38392. Imho it goes in the same direction and we can continue there.