During the import operation, an admin has to choose how to handle HTML filtering. Here are additional details.
What is the unfiltered_html Permission in WordPress?
This unfiltered_html permission could be very dangerous in the wrong hands. Do not give this permission to any user you do not trust. WordPress has disabled this permission for most users because they rarely need it.
If you just want those users to be able to insert content like YouTube video iframes, use WP’s built-in embed shortcode to embed that content instead.
Reference: Add unfiltered_html Capability to Admins or Editors in WordPress Multisite
Learn more about how to enable the unfiltered_html if you decide to do so: What is the unfiltered_html permission in WordPress?
WordPress HTML Filter
What is the force_filtered_html_on_import filter in the WordPress?
This filter determines whether to filter imported data. Multisite uses this hook to filter all data by default, as a super administrator may be assisting an untrusted user. Other plugins or custom code can set this filter as well.