Hmm, that would explain why this happens and would even be consistent. And now, I cannot tell anymore if this was the reason for missing alerts for me. I'll try to test it.
However, this would not solve my problem.
Pinging @Ultramanoid - just in case you want to add something.
Using web browsers here, naked browser Pro and Firefox on android and Firefox on PC. Starting a few days ago I noticed a lag between the time a new post or activity was performed and when it was reflected in the indicator in the top bar of the webpage. Starting yesterday, when you and others mentioned it there seems to be a complete failure of notifications even in web browsers. Today I've been catching new replies by manually browsing threads and viewing list of watched threads. Just now there was a dynamic new post notification along with indicator at the top but I've not seen enough of that to say this is fixed.