I agree that, if knowing about the afterlife is made likelier by being an experienced and powerful wizard, Dumbledore should be expected to know about the afterlife. However, we have now gone from "it's everybody's observations of the world" to "it's Harry's observations of the general public" to "it's Harry's observations of Dumbledore". In other words, Harry's (and our) evidence base for the lack of an afterlife keeps getting narrower the more we think about it.

In addition, it's worth noting that Dumbledore, for all his virtues, is also great at self-deception and confused thinking (plotting and strategy excepted). There are all manner of circumstances under which Dumbledore would be unaware of the existence of the afterlife - for example, if it led to a conclusion he was unable to accept, all his power and experience might not stop him flinching away.