I’ve been seeing this issue with the disappearing notification as well, but not consistently. Just updated 3 Manjaro Cinnamon machines. It’s gone on one, but not on the other 2. Although on one of those two, it took a couple of reboots to re-appear.
I tried the trick with disabling/enabling it at startup, but that didn’t work for me.
My theory was that it will reappear by itself the next time there actually are updates.