Hi Juraj, I didn't even know this was that customizable. It turns out DocumentModifiedWhen wasn't checked as Searchable. I'm now able to sort by that field, which is great.
I have another issue that seems to be related however. Having a sort value on the Smart Search Results breaks the relevance score (<%# Eval("score") %>). It returns NaN. This worked fine before the 8.2 upgrade.