I’m about 95% certain that the problem arises due to the detection/conversion/thumbnail step (and the fact that we disallow embedded media here). Speed is only important in that it’s an asynchronous process and if you paste a link quickly and then submit, it’s likely that the process has not finished yet.
I’m also about 95% certain that if you post, and the asynchronous process completes (which you can see happening dynamically), and then edit your post, it will then not succeed, because the once the thumbnail gets cached, the converted URL comes up immediately rather than after a second or two.
If you need to edit your post, you can try one of the alternate URLs, like youtu.be. You can also put a “&t=0” at the end of the URL. Discourse isn’t that smart and it caches based on the exact text of the URL. In fact you can probably just append “&blah=blah” or whatever to confuse Discourse (haven’t tried it, but I think it’ll work). Just change it to something else every time you edit.