Nice little research =), we had similar issues and the workaround we used was to basically handle this in the rendering Freemarker code where you access the avatar-url. As you said, if the image-size part is stripped out, it works, therefore something like this:
<#list response.items as item>
<img class="avatar" src="${item.avatar.href_medium?keep_before('/image-size')}" alt="" />
</#list>should do the trick. But I agree that this should be fixed in the product, especially if SVGs are technically allowed (which should of course be the case in times of retina displays)
