This suddenly started happening to me this week. Nothing I do nor change seems to fix it. Similarly I have tried remaking the browser source. I've tried making a new alerts box. I spoke with Twitch support chat two times and received zero actual help and a constant reply that its OBS or "not Twitch's fault."

I suspect that there is some issue with the alerts reaching the dashboard.twitch.tv/widgets/alertbox page so nothing is actually getting through. Twitch support doesn't know how to check this or just do not care. Similar to OP support doesn't read nor listen when describing the issue and just copy/pastes the faq documentation for setting up alerts.

Thank you, I was able to receive the test notification as well.
One step forward!
I've heard that the test notification only arrives but not in the actual stream, so I'd like to check this in the next stream.

A few days ago, I contacted Twitch support with the entire situation and the steps I took.
Their response was something like "Please tell us about your situation and the steps you took."
When I told them "I'm sure I sent you all of my situation and the steps I took," they said "I don't know about services provided by third parties." lol

My English may have been hard to understand because I used a translator, but thank you for letting me know what to do.

Fixed this by going into the filters setting on the alert box in my stream manager. Then going to the second tab and turning off do not disturb.
It somehow turned itself on some day without me realizing but now that its off they work again!

Not yet.
However, only the channel points were notified.
(I had not used it before but just tried to register)

As for other notifications, they still do not arrive.
I thought I could receive them by creating a new one, but still no luck.
I am currently using Streamlabs alerts for the time being, but would like to use Twitch alerts for staging...

I have been having the same issue for the past 2 weeks. Did you figure out a solution yet?