Only partial support for Proxy redirects (inconsistency)
Please see #4967 (closed) (Enable proxy rewrite support [P] for apache servers) - the issue was extracted from a forum post about missing Proxy support for subdomains. Unfortunately Proxy support was added for TLDs only.
The fix for TLDs looks super simple (bb00dc2d) - in this case it's just a change to the UI:
Tested with ISPConfig 3.1.13