Unlike all other Tomcat valves, RewriteValve does not set asyncSupported=true by default, so asyncSupported="true" must be explicitly specified in XML.
Many of the other utility valves can be asyncSupported by default. This is an enhancement since the valve has to be declared in the xml already so adding asyncSupported is easy.
The default has been changed to true in: 9.0.x for 9.0.0.M10 onwards 8.5.x for 8.5.5 onwards 8.0.x for 8.0.37 onwards