The sampleDensity() function relies on a pretty heavy precomputation, that we only perform once at load-time. Image attribute samplers (no matter if they're at evolve or at spawn) will only lookup the initial (non-overridden) image density lookup table.
I noticed the documentation does not mention this, so I've just updated it.
There's a task to implement this in our backlog. If you drop us a mail at support at popcornfx dot com, we can notify you when it has made it into a build.
(On a side-note, attribute-samplers at evolve should work even without PageLocalization. Enabling PageLocalization will just help performance if you have multiple instances of the effect).