anchor-center was explicitly added to (justify|align)-items.
But these will never apply to abspos boxes, because (justify|align)-self: auto for abspos boxes do not refer to (justify|align)-items at all:
Behaves as normal if the box has no parent, or when determining the actual position of an absolutely positioned box.
So it seems only relevant when we compute the hypothetical box, where it ends up becoming center (Since we're pretending to be static-positioned).
Wanted to confirm if this is the intended behaviour.