This is happening because the web app relies on the server to tell it whether or not a link is an SVG. When you first create a post, the web app briefly displays a pending post that wasn't sent by the server, so it doesn't know yet that the image shouldn't be displayed. Once the post is properly created, the server will tell the client that the image is an SVG, so the client knows not to display it.
1) Go to System Console > Site Configuration > Posts and set Enable SVGs to false
2) Post an SVG as a markdown image (like `![svg](<link to SVG>)`)
Observed: The SVG is briefly visible to the user who made the post. No other users see the SVG.
Expected: The SVG isn't visible to any user.