I am not up on these Pro Boards, but I am assuming there was some sort of update that made Youtube links in signatures auto-inbed.
@thecharizardauthorty asked about it so I investigated.
You can take away the “www.” and it will stop the auto-inbed, but unfortunately that makes your link unclickable. I tried youtu.be links and a few other variations but it didn’t work.
EDIT: Use the ‘Insert Video’ option when you are doing your Signature(or premake it in a post) to get the Hypertext’d link instead of inbedded.