Without surprise, I have the same result here with Win10PE x86 build 10586 (SE or Not)
and it operates again, indeed, with 10295 and following or with 10240.
no problem with Win10PE x64.
However, it works with real Win10 x86 build 10586.
I guess it is probably related to something in original boot.wim System Registry but what!
About creating Mount point, I noticed that the native mount command from the context menu works, or also imdisk or subst.
Thanks to PassMark team for testing