I see this all the time and have assumed it's related to difficulties correlating outbound messages (to the spammer) with responses. Also within a message thread the service sometimes fails to respond to the most recent email from a spammer. An example thread showing both issues: https://spa.mnesty.com/conversations/caabzkyg/
The failure to respond is because I set a limit, it stops after a few tens of messages to avoid endless loops with bots. The other messages are because it gets put into a list and just gets spammed, as far as I can tell.
[1] https://spa.mnesty.com/conversations/vkeezpyz/