Worked perfectly! Thank you!

Insert this CSS code in Custom CSS Code Field from Dashboard > Forums > Settings > Styles Tab

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#wpforo #wpforo-wrap .wpfa-item.wpfa-video{
    min-width: 300px;
}

Something sets wpForo videos width=0 and height=0 and that function adds the data-orig attributes. It's obvious that the wpForo videos are affected by something. We've nothing to fix in the plugin for sure, we just need to find the problem maker and try to prevent it doing that. So please attach the theme you use and keep disabled other plugins during the test. Make sure the data-orig attributes are not added by other plugins.

It seems I made a mistake. I assumed it was the video tag that was causing the issue. After further checking, I see that the video tag is on other videos on the site and some of them are also set to 0 but are still able to go fullscreen and return properly. See the attached video below:

Do you still want me to zip the theme I'm using?

Thank you for the detailed explanation of the issue. If you check the wpForo Addon Video tag you'll see that it doesn't add any extra tags like data-origwidth and data-origheight. These two attributes are being added by some JS code from the theme. These are not standard attributes of the video tag: https://www.w3schools.com/tags/tag_video.asp

The video tag doesn't have such attributes, and they are custom attributes for custom JS functions. And the JS code which dynamically inserts this attribute is located in your theme. it doesn't matter who is the developer of the theme, but the theme should not add extra unknown attributes to all video tags. wpForo doesn't have control over the JS functions of your website, it's almost impossible.

Let's do some test. And find the two attributes inserting functions origin. Change the theme and check the video tag, if you still see the same attributes, then this comes from other plugins. Also, please zip the main theme you use and attach it here, so we could do a deep debugging.

I understand what you're saying but the theme I am using is a theme provided by Wordpress, Twenty Twenty. The theme I switched to was also a theme provided by Wordpress, Twenty Twenty-One. Therefore, the problem cannot come from Wordpress's own theme's JS function but from wpForo's integration. 

I've attached a video below that shows the problem. Wordpress Twenty Twenty's videos are able to go fullscreen and return properly everywhere on the site, even in the wpForo forum, except for in the wpForo Forum when embedded using wpForo embed functionality. 

I see you changed the theme and the video disappearing issue is solved. So I was correct, the problem comes from the theme JS function:

https://gvectors.com/wp-content/uploads/2021/09/video-problem.gif

Please let me know what other issues you see on the new theme?

Please read my previous post again. I already said this is either a plugin or a function of your theme.  So if this is not a plugin, then this is the theme built-in function. Please find that function setting and disable it. This is the reason of disappearing the video player after minimizing. And this could be a reason of other issues. So again, go through all options and settings of your WP theme and search for video embedding and video size controlling / fitting options and disable them.

Once the video disappointing issue is fixed, we'll take a look at other problems.