I've just had a look now (21:24 BST) and been able to add something no problem on my iMac, which in turn is showing up on my phone straight away too, so I'd hope it should be working again for everyone 🤞 However, I will admit to wondering "for how long?"
I don't believe these threads are monitored, so the only way to make sure Apple are seeing this is to raise it with them as a bug report each and every time it happens: Feedback - Apple Music - Apple. Might be worth giving them the URL of this thread every time too, i.e. so they don't default to the usual assumption that it's user error/a local issue! 😆