Alertmanager pod msg="dropping messages because too many are queued"

hello,

What did you do?
I'am using alertmanager in a kubernetes pod, it's connected to Prometheus, Karma and Kthnxbye to ack alerts.

What did you expect to see?

normal memory usage as before

What did you see instead?

Recently, the memory usage graph of alertmanager is experiencing a linear increase.
In the alertmanager logs, I have this message:
level=warn ts=2020-12-17T09:32:04.281Z caller=delegate.go:272 component=cluster msg="dropping messages because too many are queued" current=4100 limit=4096
Rule expression of the message :

// handleQueueDepth ensures that the queue doesn't grow unbounded by pruning
// older messages at regular interval.
func (d *delegate) handleQueueDepth() {
	for {
		select {
		case <-d.stopc:
			return
		case <-time.After(15 * time.Minute):
			n := d.bcast.NumQueued()
			if n > maxQueueSize {
				level.Warn(d.logger).Log("msg", "dropping messages because too many are queued", "current", n, "limit", maxQueueSize)
				d.bcast.Prune(maxQueueSize)
				d.messagesPruned.Add(float64(n - maxQueueSize))
			}
		}
	}
}

Please note that there is no action to justify this increase.

Environment
Alertmanager : v0.21.0
Prometheus : v2.18.2

Start at delegate.go:272 and the handleQueueDepth entry point, then inspect the Alertmanager pod logs and memory graph alongside the reported queue values. Determine why messages are accumulating despite the reported setup; done means a confirmed cause and a scoped fix or clear explanation of the expected queue behavior.