All games that supports Monitor Distance 0% (BF1/BFV, Apex Legends, MW 2019 etc) uses the vertical axis, i.e. MDV. It's also more consistent if you go from e.g. 16:9 to 21:9, as MDV will not change (it will keep the 16:9 portion of 21:9 the same), while MDH will change and essentially stretch the scaling to fit the increased width.