https://drafts.csswg.org/css-anchor-position-1/#typedef-position-try-fallbacks-try-tactic isn't particularly clear about whether flip-block means "block axis in the box's writing mode", or in the containing blocks' writing-mode.

I think WebKit at least uses the box's writing-mode, but given position-area by default operates on the containing-block's WM, that might be more consistent?