When the ec2-fleet-plugin adds new spotfleet instances as Jenkins nodes using the Launcher selection of "Launch agent agents via SSH", the nodes all connect just fine, but some percentage of them disconnect with a SEVERE I/O error shortly after launch. The I/O error happens at exactly the configured "Connection Timeout in Seconds" from launch. When reconnected after that, they have no problems.
I have not confirmed, but I think this only happens when "Max Idle Minutes Before Scaledown" is set (attaching the IdleRetentionStrategy to the node).
Here's the sequence from the logs with the default ssh connection timeout of 210 seconds:
15:01:11.832 - INFO: Found new instances from fleet (ec2-fleet test): [<snip>, i-08db665d464785aec, <snip>]
15:01:21.966 - INFO: Idle Retention initiated
15:01:21.967 - INFO: Attempting to reconnect i-08db665d464785aec
15:01:56.067 - SSH Launch of i-08db665d464785aec on 10.21.131.211 completed in 34,083 ms
15:04:51.990 - SEVERE: I/O error in channel i-08db665d464785aec