Docker BuildKit build with tmpfs mount fails the 2nd time around

Question

This works:

# note: cache .cache/go-build across docker builds
RUN --mount=type=tmpfs,target=/home/myuser/.cache \
    pacman -S --needed --noconfirm go &&                                \
    su - myuser -c "                                                   \
        my GO build goes here" &&                                           \
    pacman -Rcsn --noconfirm go

in the sense that the non-root user myuser is able to write to the tmpfs mount and the GO build completes successfully.

But if I prepend to the above, another instruction in the dockerfile which mounts the same tmpfs, such as

RUN --mount=type=tmpfs,target=/home/myuser/.cache \
    ls -Al

then surprisingly the GO build fails with

#18 42.37 failed to initialize build cache at /home/myuser/.cache/go-build: mkdir /home/myuser/.cache/go-build: permission denied

It appears as though the tmpfs mount does not have proper permissions the 2nd time it is mounted. Anyone experience the same? is it a bug?