Hi :
I only tested PG-12-Beta 3.
Could you try testing a released version (like 11.5), between the same two machine? It could help rule some possibilities in or out.
Also, what are all your timeout settings? And your keep alive? And your unix_socket_directories?
select name,setting from pg_settings where name like '%keep%' or name like '%timeout%' or name like '%socket_direct%';
I suspect that something in your network (firewall, gateway, etc.) is interfering with the connection. Turning on compression just slows down the client (pg_basebackup) enough that it gives this interference time to happen.
Backup completed normally, but did not seem to end correctly.
In your first email, the server did think something went wrong:
"2019-08-12 18:20:03.417 CST [11446] LOG: could not receive data from
client: connect timeout"
I think the "connection timeout" part of this message is generated by your OS, and PostgreSQL just passes it along. It doesn't seem to be a timeout which PostgreSQL is expecting to happen, which makes me think the problem is in your network rather than in PostgreSQL itself.
In the log in your most recent email, it does look like the server believes everything completed normally, while only the client senses an error. Again, it seems to me that something in your network is injecting itself between the two ends of the connection.
Cheers,
Jeff