iframe in WP MU

Hi,
I’ve recently found a problem with iframes in Worpress multisite. When I try to insert an <iframe>…</iframe> code to Post or Page in HTML editor as a local admin (not superadmin) or editor, it dissapear just after I click update. When i try it as superadmin, it normally works. Tested on 3 different instalations… one of them was new clear.

Any ideas where could be a problem?

Thanks