Long running backup preventing auto vacuum

Hi,

PostgreSQL version - 11.7

We are using wal-g for backup/restore. We had a duration of high WAL
generation due to application usage due to which archive backup was
delayed.

Backup was started through cron schedule and it didn't complete because it
waits for the last WAL to be archived. Now, I know that long running
transactions will prevent vacuuming of dead tuples but in this case we were
able to run manual vacuums successfully but auto-vacuum never starts.

As soon as we terminated the long running backup session, the auto-vacuum
started kicking in.

Wanted to understand why a backup operation is blocking the auto-vacuum?

Wal-g uses non-exclusive backups and waits for the last wal to be archived.

Thanks,
Nikhil