ok I added the ProxyPreserveHost directive - reconfigured hosts and all is now good without the workaround.
I struggled to find any doco on this... I will write up some sample configs and explanations and pass back for tidy up and inclusion in your doc as you see fit for others following. Specifically since apache doc recommends against the use of the this directive, it would be good to document it for others.

yeah thats a load balancer style setup I'm just using apache mod_proxy and ProxyPass. (I have about 5 apps going through same proxy so far) I'll have a go at adjusting that behaviour.

the reverse proxy is on a different server. It responds to phabricator.domain.com so if it tries to proxy to that address it will respond itself in a loop and not get to the internal machine.

That is not necessary, the only domain I am interested in serving from is the external one. internal users can go external to access it.