This is expected. NoData and options to execute it as X is related to a situation when query does not return any points. Only in this case it is classified as no data.
A situation when a series is missing is not considered as NoData. In this case, the alerting keeps the last state of the alert instance for 2 rule evaluation cycles and if it’s still missing, it evicts it from the state with transition Normal (MissingSeries).
In Grafana 12 we will let users configure that number of evaluations.
