Hi Nick,
We do not have any option to set the TTL in Destination in AAM UI, we have start and end date to send segments to Destinations.
Is that TTL on the actual destination's platform end? If yes, then mismatch in the TTL could be the reason.
I would recommend you to raise a support ticket to get this investigated. Ask team to check destination configuration settings and have a look at trend for that destination, what is the average match rate for that type of destination in general.
Thanks,
Varun Kalra