It seems like this 3rd party plugin Joomla! Admin section in Frontend injected those iframe code on the print preview, you can check my attached screenshot below.

After disabled it, it should render correctly now.

Currently I am not really sure what is the purpose of this plugin, but I already help you re-enabled back.

Hello Arlex,
Thank you for discovering this. It's a plugin that enables you to enter the backend easily when logged in at the frontend.
I'm going to deactivate it.
Best regards
Ineke

You're most welcome.

Just for your information, I have locked and marked this thread as resolved to avoid confusions in the future. Please start a new thread if you have any other issue in the future so it will be easier for us to manage your inquiries.

Thanks for understanding