Yeah, I upgraded to 8.18.0 yesterday. Same problem.
The rule refuses to perform any alert action. I am curious if others are seeing the same thing? I am sending it to an index, just does not work. Can others confirm?
I found other rule types that don't work either. Is it just me or does rule action alert usage seem inconstant if they work or not?
I created a separate post about this here Custom Threshold not triggering alert to index - Elastic Stack / Kibana - Discuss the Elastic Stack