Hi, I came across this issue after much research as it appears a fairly unused scenario.It is still apparent in 2.2.22-13+deb7u6 on Wheezy. It is not present in Apache 2.4 on Jessie.
It was fixed in apache in r1467593: https://svn.apache.org/viewvc?view=rev&rev=1467593 Details can be seen in: https://bz.apache.org/bugzilla/show_bug.cgi?id=54698 Workaround is to swap order of key and certificate in your /etc/apache2/proxy.pem file.