Help diagnosing replication (copy) error

Hi,

I'm in the process of migrating a cluster from 15.3 to 16.2. We have a
'zero downtime' requirement so I'm using logical replication to create the
new cluster and then perform the switch in the application.

I have a situation where all but one table have done their initial copy.
The remaining table is the largest (of course), and the replication slot
that is assigned for the copy (pg_378075177_sync_60067_7343845372910323059)
is showing as 'active=false' if I select from pg_replication_slots on the
publisher.

I've checked the recent logs for both the publishing cluster and the
subscribing cluster but I can't see any replication errors. I guess I could
have missed them, but it doesn't seem like anything is being 'retried' like
I've seen in the past with replication errors.

I've used this mechanism for zero-downtime upgrades multiple times in the
past, and have recently used it to upgrade smaller clusters from 15.x to
16.2 without issue.

The clusters are hosted on AWS RDS, so I have no access to the servers, but
if that's the only way to diagnose the issue, I can create a support case.

Does anyone have any suggestions as to where I should look for the issue?

Thanks,

Steve