If I'm interpreting this correctly, you have a customized xfmg_media_view.less template in a style. This is trying to include the other templates, which no longer exist. (Presumably from someone hitting a cached version of the page.)
I found the templates, including xfmg_media_view.less, they are included in my current style. However, I don't have any revert option and searching through the style, I can't find any reference to XMFG, not sure what calls it. Any ideas?
The template is being called directly by a CSS call (presumably from a view of a cached page).
Can you screenshot the template list showing the xfmg_media_view.less template? It may be that it's been customized in a different style (potentially a parent).
Yeah, it would need to be reverted at the level where it's customized -- you seen see the icon on those. Alternatively, you can customize the template in your child style and simply empty the contents.
This issue is coming up because there is a cached page that's making a valid call to get the CSS. We add a unique key to validate that the CSS being requested is a legit request. Because that matches (since it did come from a valid page), we build the CSS as requested, which then tries to load the named CSS which does exist because it has been customized in your style. The way to avoid this would be to not have the customizations for the XFMG add-on included in your style without it being installed. (It also wouldn't end up happening without a previous working XFMG install as there wouldn't be a valid CSS link including this template.)