Your description is confusing: since the cache is an "in memory" data structure, how can AuthDBI find the an entry in the cache at the first request? Please post the output from the debugging.
I know it's confusing, that's why I'm asking.
Normally, you'd expect the cache to be cleaned ( well destroyed ) but it isn't.
I'll add the debug output in a sec.
23912 Apache::AuthDBI::authen cache: found >xxxx,dbi:mysql:web:127.0.0
+.1:3307,tAuthUser,aut_s_login< >yyy< >time< >xxxx<