Iframe being removed only for some users when publishing a page

To post most HTML tags (such as <script> and <iframe>), the user requires the unfiltered_html capability. In WordPress Multisite, this capability is (by default) assigned only to Super Admins (ie, those that can administer the entire network).

This is by design, and is due to security concerns, among other things:

Note: Enabling this option for untrusted users may result in their posting malicious or poorly formatted code.

Reference