Dear DW_B,
Thanks for your updates and I'm following up on the thread with you.
As the Upload File type question doesn't support the Anyone access in Microsoft Forms, I'm afraid that you may be required to log in with your account after session expires when you use the embed forms.
I'm afraid that PowerApps may not achieve your requirement as the users need the PowerApps license to use the form. You may first add the external users to your Azure AD in your tenant and then assign the PowerApps licenses to them for them to access the form.
So the PowerApps solution may not be applied to your environment if you haven't enough PowerApps licenses for external users and internal users.
Based on my test, you should be able to get the form format again after you refresh the page. As you don't encounter such an issue in another embed form, I'd like you to change the form like the following to enable the Anyone link for external users and internal users to fill and embed the new form on your page to see if the issue disappears in your environment. If yes, I suggest you use the way to move forward.


You can use the embed code as follows.
<iframe src="Anyone Link" style="border:0px #ffffff none;" name="myiFrame" scrolling="no" frameborder="1" marginheight="0px" marginwidth="0px" height="400px" width="600px" allowfullscreen></iframe>
Thanks for your understanding and have a nice day!
Sincerely,
Cliff | Microsoft Community Moderator