Hi Falco, thanks so much for such a prompt response - really appreciated 
Sure… not sure if you want the URL of the posts on our form or of the specific SketchFab URL, so I will provide both…
Here is a link to a post where I was testing to see what was happening, using URLs from our original posts (which worked) and then more recent ones (which didn’t)…
https://community.gamedev.tv/t/sketchfab-test/11802/2
You’ll notice on that post that the second item down (the police car) is thumbnailed… yet if you follow the link in the text below it where it says “Original Post”, you’ll see that the very same URL produces a large version of the 3D model embedded into the post itself (desired behaviour).
This is the specific URL for that model on the SketchFab site:
hmm… ok, your forum is rendering it now as a thumbnail also… erm… hang on…
https://sketchfab.com/models/303974fd05a74dd184e6376b995cd7f6
Right, at least you can see the full URL if I do it that way…
What I’m not sure of is whether it’s a change in functionality via Discourse (which may explain why older posts are still ok), or whether it’s a change at the SketchFab end (but that wouldn’t explain what’s going on with this police car specifically, where it’s ok in an older post and not in a newer one).
Again, not being an Admin I can’t provide an details of settings from our config - sorry.