I just ran into this when using a backup script that uses mysqldump --verbose which generates a log of dashes in stdout. It was extremely confusing to track down the cause of the 1 persistent warning that kept displaying after otherwise successful backups.
Reading this post I now see that the warning was not from my backup script, but from the output being interpreted as a massive argument to the following backup command. This behavior is very unexpected and not documented in the scripting section as far as I can see.
Is there a good reason the stdout from a backup script is being used downstream in the backup command? If nothing else there should be a setting (maybe default on) to just log stdout from any scripts so they don’t impact the backup command.