I noticed recently that video links that are put inside of a spoiler tag do no properly embed when you post the message. However, when you are typing the message, they embed perfectly fine in the message preview. Just to provide an example, I will put a video link in this message, along with a picture of the message preview.
Ok for some reason it seems to be working now. I wonder if it is just when combined with a large number of photos inside the spoiler tag. I noticed it while creating a suggestion, so I will copy one of the spoilers from that suggestion into here.
Yes that does indeed seem to be the issue, I edited the post.