In moving from the 2.8 codebase to the current master (also from MP to Rex), we've found shadows are no longer cast from topography. I found there's a new (relatively speaking) terrain option, cast_shadows, but setting that to true seems to have no effect. Am I missing something else?
Since terrain shadows never really worked that well to begin with, we disabled them by default. You can still get the old buggy behavior with the cast_shadows property (which works for me - make sure you are zoomed in far enought). But it still looks bad IMO.