TBH, I'm not quite sure what happened. The SMF plugin that we were using was very old and outdated so I replace it with a plugin that was still being updated. I've actually been meaning to do this for quite some time.
On a side note, I looked into how to change the behavior of embedded videos when they are quoted but it looks like I would have to replace the auto-embedding nature of them and make it so that you needed to wrap the URL in a bbcode (something like [video]VIDEO_URL[/video]) for the link to embed. Are you guys alright with this? Personally, I think it would be better but it would also be more work for me. I've already tweaked the plugin once so, either way, I'm going to have to manually update the plugin whenever there is an update.
The problem of having bbcode is that all old topics(including the ones of past tournaments, which are locked) would have no embeds anymore. I can definitely see why having bbcode is better, sadly it breaks the reverse compatiblity(unless you have some kind of script which automatically adds the code)
Since I'm the webmaster of GTM, I can perform operations on the database itself instead of manually going into old threads just to add the new tags. The easiest way of fixing this would be to perform a mysqldump on the database to get a single file representing the entire forum, open it up in Notepad++, use a regular expression to add the bbcode around all video links, and run mysqlimport to add it back to the database (after deleting everything that is already there). Sadly, I would have to switch the forum into maintenance mode while I did this so that posts wouldn't get lost in the process.