Unsure of root cause, but an agent crashed and then each attempt to bring it up had it crash again generate a core dump.
The last line of the logs the agent noted was:
20211130/205228.960 - U00063085 FT '949649565': File transfer with partner 'AGENT_NAME' started - sending.
Once the AGENT_NAME agent from the message was disconnected that agent from the AE, then everything came up correctly.
Release : 12.3
Component : AUTOMATION ENGINE
This is fixed in the following UNIX agent releases:
The issue was caused by a buffer overrun which happened when the host name of the File Transfer partner could not be resolved by getaddrinfo() due to an incorrect DNS configuration.