The fix is to add nocanon to the proxy config.
ProxyPass / balancer://frontendcluster/ stickysession=ROUTEID nocanon
ProxyPassReverse / http://10.1.1.1:9001/ nocanon
ProxyPassReverse / http://10.1.1.2:9001/ nocanon
Normally, mod_proxy will canonicalise ProxyPassed URLs. But this may be incompatible with some backends, particularly those that make use of PATH_INFO. The optional nocanon keyword suppresses this, and passes the URL path “raw” to the backend. Note that may affect the security of your backend, as it removes the normal limited protection against URL-based attacks provided by the proxy.