First, why are you using an iframes? Especially as your code indicates the iframe source is a part of your site.
If you are using it to display someone else's site/page, then they may have included code to stop you doing so, I insert such code after a few months, (I like to give people time to ask permission to do so first).
Always post link to problem page, otherwise we are guessing
It's a separate page with a long list I want nested inside a tab, and scrolling.
There is one functioning correctly on another page on my site.
The page is in development, not live.
Make it live
You don't need an iframe for this. To make a scrolling division or table cell, all you need is a CSS height value and the overflow property.
height: 300px; /**height in pixels**/
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