Setting `FROM --platform` pointing to a stage is silently ignored

I naively tried something like this in my project:

FROM alpine AS base

FROM --platform=${BUILDPLATFORM} base AS expensive
RUN sleep 100 && uname -a > /foo

FROM scratch AS final
COPY --from=expensive /foo .

My expectation was that the expensive stage would only run once on my native platform. However, I see it running twice in parallel

This however only runs the expensive stage once:

FROM --platform=${BUILDPLATFORM} alpine AS expensive
RUN sleep 100 && uname -a > /foo

FROM scratch AS final
COPY --from=expensive /foo .

My guess is that --platform only applies to docker image sources? Should it be an error if used with something else?

Start by reproducing the two Dockerfile examples in the issue and compare how the stage-based and image-based FROM --platform forms are processed. Trace the Dockerfile frontend handling for FROM and stage references; done means the behavior is either corrected or an explicit error and regression coverage are established for the reported case.