Convergence is probably not that uncommon when it comes to guides, fixtures, and so on. Made myself a dovetail marking support similar to yours; only not as good.
Lessons learned: form plywood is very stable, but oh so “slippery”, and an edge support all along the vertical part, forces the support towards the edge of the vice, which further increases the risk of the marking going awry.
My current contraption (photo attached to a previous post) is a bit elaborate to set up, but keeps the work pieces in place, and compared to my setup for wide concealed mitred dovetails, it’s really nothing at all.
Also noted what seems to be an identical approach to prevent compressing the wood when chopping out