per your testing on 4.0.18, it looks like set
in httpd.conf is not same as setting CacheRoot at admin console.
and as I tested, without "CacheRoot /lscache/" in httpd.conf, but set it in admin console, rewriterule + cache will work. so it looks that
"CacheRoot /lscache/" in httpd.conf +
set "Cache Expire Time" in admin console can enable "rewriterule + cache"
this should not be designed behavior and complicated to users. please try 4.0.19 to see if this has been fixed.