iframe auto sandboxed ?

Try adding the sandbox attribute to your iframe tag with allow-same-origin and other permissions your iframe requires. This may help you regain the ability to use sessionStorage within the iframe.

<iframe src="your_iframe_src.html" sandbox="allow-same-origin allow-scripts"></iframe>

Visit edge://flags in the Edge browser and search for flags related to iframes or sandboxing.
(Look for anything experimental that might relate to enhanced sandboxing or security restrictions on iframes. While this may be a temporary fix, you can disable any experimental flags related to iframe security to see if it resolves the issue.)


Submit feedback directly to the Microsoft Edge team. You can report issues through the Edge feedback tool by navigating to Settings > Help and feedback > Send feedback or by using the keyboard shortcut Alt + Shift + I. Provide as much detail as possible about the issue and how it affects your setup.


ALTERNATIVE SOLUTION:
PostMessage API: Use window.postMessage to communicate data between the iframe and the parent window, then store it in sessionStorage on the parent.
Local Storage in Parent Window: If feasible, store data in localStorage within the parent window and pass the data to the iframe when needed.


Note: This behavior might be an experimental feature in the Dev version, keep an eye on the Edge release notes for future updates.