I designed this element where it is white on normal and green on rollover. First I created a circle, then I created a template of a projector that I laid over it. In design mode it is fine, in preview page in browser, it is an oval?? WHY?
What did you create? A PSD file?
Might be that you ran into the annoying "vertical-shift-bug".
But first we need more information, please.
Don’t think, that is any kind of bug. It is „mis-layouted“.
Please place the objects of your screenshot into a newly created .muse file and share it with us. Then we’ll see.
Please use these instructions: https://forums.adobe.com/docs/DOC-8652