Wiley Horne
Derek,

You’re bringing to our attention the big advantage of BU planes in mitering, where the edge is subject to less unsupported shear than in the BD case. So much so that A2 does not chip in your testing. The result with A2 perhaps underlines the advantage of the BU geometry for mitering, based on the line of force and the greater support of the edge.

After all, the BU plane was developed in Britain and Germany back in the day, specifically for mitering.

Wiley