All I was trying to say is the inconsistent UX that changes if one image is select or multiple isn't friendly i.e. pressing T on a single image tags/untags but on multiple images it does nothing.
I think the goal of the shortcuts is to make it so that changing an attribute on a selection of multiple items always needs a 2-button shortcut to change a star rating, color label, or tag, to reduce the chances of an errant keystroke from updating a large selection of items.
If you prefer that the the single-key shortcut for tagging a single image is consistent with the multi-key shortcut for tagging multiple images, the + key will also add a tag to a single selected image.
The behavior is a bit unfriendly, if you in the middle of a contact sheet (viewing a directory), it just resets the index to the top left image of the current view.
I tested on both Mac and Windows and cannot reproduce what you are describing. When you say "resets the index" do you mean that it selects the top left image? All that happens for me is that it removes the selection, and it leaves the Contact Sheet exactly where it was. Whether the contact sheet should stay where it is, or should move to show the next image after the selection could probably be debated, but if it is indeed actually selecting an item after you "deselect all", I think most would agree it shouldn't do that. Am I understanding correctly that for you it both selects the item at the top left of the contact sheet and scrolls the contact sheet to make that thumbnail visible?