I use the same [] code but it's odd I dont get the same effect. 
It looks like you took the url from the browser address window.
In Youtube, underneath the video there is a "Share" link that will provide you a url that will work with the bbs embedding. it's formatted differently.
Why that one has to be used, I do not know.
The link you should have used in the embedding field would have looked like:
https://youtu.be/XTPd6x8Q6yE