J!: 3.4.1
K2: 2.6.9
Tested themes: Gantry RT-Vermilion, Beez3
All caching and compression functions disabled

When using Edit Item in frontend, a modal will pop up to edit the K2 item. Everything will load fine except that CSS class ui-tabs-hide will not apply to appropriate DIVs, so all tabs are showing at once. Clicking on content tabs does not remedy the issue. Loading the iframe content in a different tab without modal functions properly. I'm wondering if this functionality is hindered by the iframe itself. Any ideas on how I could correct this?

This is most likely by a JS conflict.
Can you check the console tab of your browser's developer tools for any errors?

JQMIGRATE: Logging is active
external.php?type=djs:1 Uncaught SyntaxError: Unexpected token <
external.php?type=js:1 Uncaught SyntaxError: Unexpected token <
all.js:64 Invalid App Id: Must be a number or numeric string representing the application id.
all.js:64 FB.getLoginStatus() called before calling FB.init().
tweet_button.12a0910b1b8c2c2ad25af3fcd05d55c1.en.html:1 Consider using 'dppx' units, as in CSS 'dpi' means dots-per-CSS-inch, not dots-per-physical-inch, so does not correspond to the actual 'dpi' of a screen. In media query expression: (-webkit-min-device-pixel-ratio: 1.5), (min-resolution: 144dpi)
2all.js:64 Received message of type object from platform.twitter.com, expected a string
discovery.bundle.95eb8922131c582d2ac1597fed152e8b.js:1 [Discovery] It looks like there was a problem: Error: Not enough data {stack: (...), message: "Not enough data"}

Most of them look to be coming from 3rd party; Facebook, Twitter, Disqus, and ArrowChat. The only one I can control directly on my server is ArrowChat. I believe Twitter and Facebook are coming from the native integration with K2. I'll try disabling Disqus and commenting out ArrowChat to see what results that yields.

Okay, after dismantling the website I finally caught it. There's no obvious conflicts, but the HTML is rendering differently between iframe-induced modal and a direct link. Here's a rough example of the correct output:

And the incorrect output from modal:

And for the life of me I can't figure out why it's doing this. It makes no sense!

I looked at itemform.php and it looks to be literally interpreted in the modal. I'm unable to find where ui-tabs-hide is being generated from.

Is there a link to your site?
Also this is quite possibly caused by a 3rd party extension.
Which other extensions are you using?