I noticed this happening since a bit of time but thought it was going to be some temporary issue. I see that it’s still happening on our installation, and I have no idea why.
There is no particular setting change in our onebox section beside blocking some websites with undesirable content but every time a mp4 file is linked, I simply see that link to it. In some cases I can see this box instead:
I tested in try.discourse.com, and it seems to work instead, but of course I can’t check the admin settings over there. Is there some known issue that I can fix with admin settings tuning?