I see "Find previous Shift-F3" in the menu always disabled.
Also, the direction of F3 depends on the settings that I last time used in the Find box (which may not already be visible if it was many hours ago): depending on what I did many hours or days ago, F3 searches forward or backward (and Shift-F3 always does nothing). When there are no more occurrences, F3 silently does nothing. Usual behavior of F3 is to find next searching downward, and when there is no next, search from the top of the document.
I suggest that F3 should always search downward and Shift-F3 always upward.