Is it a bug? Well, let me ask: Imagine that your datatable referenced an external model via the property - would you be happy that a repeated request returned stale data?
Although this isn't my area of expertise, I'd imagine that if you want to do caching, you'll have to write code that caches - only you will know that the data is stale. As I said though, it's possible I'm missing something here.
Another option (in JSF2) is to use an ajax command that only updates the part of the page you want to change - this would eliminate all of the overhead you are concerned with...
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