I'm going to report these here even though I was still on 18.104.22.1685 this morning as I doubt the issue is corrected in the 3.6 release and the issue has remained through several beta releases.
I continue see no rhyme nor reason to alert popups.
Today I received no alert to for this accident on my route:
If I tap on the accident icon, the popup confirms its on my route. However, because this is not an alert popup there is no way for me to indicate that the accident is no longer there (there was no sign of an accident on the road).
Further down the road the same thing happened again:
On the other hand, I continue to receive alert popups for speed cameras when my speed is not exceeding the limit:
Regional Coordinator: Mid-Atlantic, US
Verizon, Nexus 6, Android 6.0.1, Waze 22.214.171.124