Currently, the HTML AAM maps placeholder to the accessible name computation. In no way does the name computation include placeholder. aria-placeholder maps as follows in UIA:

Expose as placeholder= in AriaProperties.

So, the HTML AAM needs to change the UIA mapping. to refer to the ARIA aria-placeholder mapping for UIA and not the name and description computation document. The ARIA spec. also states that placeholder wins over aria-placeholder when both are present - consistent with the HTML AAM spec.