I checked your site there is a Javascript error when I looked the console that might be causing this. If you have added some custom code to the site that may have caused the scrolling issue, kindly try removing the code then check the site again.
Please check this link on how to use the Google Chrome console to check for Javascript errors:
Also, try adding the iFrame to a different page and see if the issue happens. If you are seeing the issue, it could be something that is native to the iFrame code that you have.
Lastly, you might want to check and try the suggestions from this here:
That custom code will require custom development work which falls outside the scope of support we can offer. I recommend you to contact to a 3rd party to help you with this.