Thanks, Ruslan,
Thing is, I tried to "debug" whether $dontcache is being set at all by exposing it via response headers (along with content-length), and it shows that $upstream_response_length is ignored by map completely, i.e. no matter where I use $dontcache, it will never get any value different from default (i.e. 0). Even though $upstream_response_length is validated correctly (and can be exposed in headers), the map directive just ignores it.