This is just how IPBoard works, there's no security issues present as it takes the HTML elements from your reply and basically strips it of everything but whitelisted attributes (as determined by the Invision Power). Also, there was no custom CSS in your reply, just a element style with height and width - these are used by embedded images for example.
If anyone abuses how IPBoard works to intentionally cause things similar to how your posts ended up, they can be dealt with by staff under 15.1 / whatever other relevant rule.