Alertmanager loses in-memory alerts when it restarts

What did you do?
Alertmanager stores fingerprints of current alerts in the memory send via clients. It stores nf-logs and silences in the disk(if configured).
When alertmanger starts it reads nf-logs and silences from the the disk. The in-memory store is recreated when the alertmanager restarts. Now, if the client has stopped sending alerts to alertmanager while alertmanager was down, the AM will not be able to send resolve notifications to receivers.

What did you expect to see?
AM should be able to send resolve notification after it restarts.

Can we also store the in-memory alerts onto the disk via snapshot(similar to nf-logs and silences) and/or while graceful shutdown and recover them from disk when AM restarts?

Start by tracing Alertmanager's in-memory alert store and the existing disk persistence paths for notification logs and silences. Investigate how startup and graceful shutdown handle state, then define and test recovery so alerts that stop arriving while Alertmanager is down can still produce resolve notifications after restart.