Adding iframe makes page content invisible

Hi, 

I'm trying to add an iframe to my clients website as they use an external system for their gym timetable/booking system. I'm using a code block, when I add the code to the code block the iframe appears but when I view the site the entire page content is invisible. 

I know it is still there because I can edit the page and hover of the right spot and edit the content, remove the code and everything becomes visible again. 

Any idea what is going on? - this is the iframe code I was supplied.

<iframe src="https://widgets.mywellness.com/facility/samsararunawaybay" allow="fullscreen" width="100%" height="100%" />

Thanks,

An iframe element is not a void element. It needs to have a closing tag.

<iframe></iframe>

Let us know how it goes.