I think this is a bug but I don't know enough about svg images to be sure.
On export, links to svg images produce an <object> element rather than an
<img>.  Here's the docstring of the function (from ox-heml):

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Return \"object\" embedding svg file SOURCE with given ATTRIBUTES.
INFO is a plist used as a communication channel.

The special attribute \"fallback\" can be used to specify a
fallback image file to use if the object embedding is not
supported.  CSS class \"org-svg\" is assigned as the class of the
object unless a different class is specified with an attribute.
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Neither chrome nor firefox can display these objects.  On the other hand,
if I modify ~org-html--format-image to~ treat svg images just like other
images (in an <img> tag) then the image displays just fine.  Is there a
reason we use the object tag instead/ If not, what's the best solution? I
na try to work on a proper fix.

Thanks,
Matt