Apache 2.2->2.4 upgrade broken pass-through[PT] after mod_rewrite - mod-rewrite

Question

Our server has been upgraded from Apache 2.2.32->2.4.20, and with that change, my mod_rewrites don't pass-through to Tomcat endpoints any longer.
Here is the Tomcat Load Balancer config:
<Location /balancer-manager>
SetHandler balancer-manager
</Location>
<Proxy balancer://tomcatHttpCluster>
BalancerMember http://localhost:9946 loadfactor=100
</Proxy>
And the rewrite rule of interest:
RewriteCond %{REQUEST_METHOD} POST [NC]
RewriteRule ^/catalog/preferences$ /ac/rest/preferences [B,PT,L,QSA]
And also the Location config which applies to all /ac requests:
<Location /ac/>
ProxyPass balancer://tomcatHttpCluster/ac/ stickysession=JSESSIONID
...
</Location>
If I go directly to the /ac Tomcat endpoint, it works. The Apache log:
mod_rewrite.c(477): [client 10.20.3.63:50485] 10.20.3.63 - - [tesla/sid#caa5f8] [rid#f75c230/initial] pass through /ac/rest/preferences
mod_proxy_balancer.c(73): [client 10.20.3.63:50485] canonicalising URL //tomcatHttpCluster/ac/rest/preferences
mod_lbmethod_byrequests.c(95): AH01207: proxy: Entering byrequests for BALANCER (balancer://tomcathttpcluster)
mod_lbmethod_byrequests.c(142): AH01208: proxy: byrequests selected worker "http://localhost:9946" : busy 0 : lbstatus 0
mod_proxy_balancer.c(614): [client 10.20.3.63:50485] AH01172: balancer://tomcathttpcluster: worker (http://localhost:9946) rewritten to http://localhost:9946/ac/rest/preferences
proxy_util.c(1783): AH00924: worker http://localhost:9946 shared already initialized
proxy_util.c(1825): AH00926: worker http://localhost:9946 local already initialized
mod_proxy.c(1159): [client 10.20.3.63:50485] AH01143: Running scheme balancer handler (attempt 0)
mod_proxy_ajp.c(738): [client 10.20.3.63:50485] AH00894: declining URL http://localhost:9946/ac/rest/preferences
mod_proxy_http.c(1903): [client 10.20.3.63:50485] HTTP: serving URL http://localhost:9946/ac/rest/preferences
proxy_util.c(2158): AH00942: HTTP: has acquired connection for (localhost)
proxy_util.c(2212): [client 10.20.3.63:50485] AH00944: connecting http://localhost:9946/ac/rest/preferences to localhost:9946
proxy_util.c(2421): [client 10.20.3.63:50485] AH00947: connected /ac/rest/preferences to localhost:9946
mod_proxy_http.c(1374): [client 10.20.3.63:50485] Status from backend: 200
mod_proxy_http.c(1048): [client 10.20.3.63:50485] Headers received from backend:
mod_proxy_http.c(1051): [client 10.20.3.63:50485] Server: Apache-Coyote/1.1
mod_proxy_http.c(1051): [client 10.20.3.63:50485] Content-Type: application/json
mod_proxy_http.c(1051): [client 10.20.3.63:50485] Content-Length: 218
mod_proxy_http.c(1051): [client 10.20.3.63:50485] Date: Fri, 28 Oct 2016 00:32:26 GMT
mod_proxy_http.c(1646): [client 10.20.3.63:50485] start body send
proxy_util.c(2173): AH00943: http: has released connection for (localhost)
mod_proxy_http.c(1791): [client 10.20.3.63:50485] end body send
mod_proxy_balancer.c(669): [client 10.20.3.63:50485] AH01176: proxy_balancer_post_request for (balancer://tomcathttpcluster)
BUT. If I try to use the rewrite URL - the rewrite happens...then nothing:
mod_rewrite.c(477): [client 10.20.3.63:50457] 10.20.3.63 - - tesla/sid#c6a5f8rid#dc2a110/initial RewriteCond: input='GET' pattern='GET' [NC] => matched
mod_rewrite.c(477): [client 10.20.3.63:50457] 10.20.3.63 - - tesla/sid#c6a5f8rid#dc2a110/initial rewrite '/catalog/preferences' -> '/ac/rest/preferences'
mod_rewrite.c(477): [client 10.20.3.63:50457] 10.20.3.63 - - tesla/sid#c6a5f8rid#dc2a110/initial forcing '/ac/rest/preferences' to get passed through to next API URI-to-filename handler
I'm no expert at Apache (until now!), but I did try changing the mod_rewrite flags to just [R] and that worked fine. I am not able to make that change on the server because that code is not under my control. I assumed the [PT] rewrite flag would send the rewritten url back through the mod_proxy_balancer and voila, but that doesn't appear to be happening.
Is there something obvious that changed from 2.2 to 2.4 that would cause this? Help! I've been stuck on this for days.

Answer 1

try to use P instead of PT in rewriterule. It works for me.
RewriteRule ^/catalog/preferences$ /ac/rest/preferences [B,P,L,QSA]