Is this an error in MS Arial Unicode?

I am testing Unicode combining characters in Arial Unicode, but when I
try U20D7 it displays the glyph (a top arrow) beside the previous
character, instead of combined with it. The same happens in fact with
all the following ranges of combining characters: U0483-U0486,
U20D0-U20E1, U3099-U309A, UFE20-UFE23.
These are all supposed to be combining characters, however they are
displayed as normal ones.
Is this an error or is it supposed to be like this?
The first range of combining characters (U0300-U0345) works OK.

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