Currently MariaBackup's stderr contains records in different formats, in addition to server-style messages.
Some lines come with a timestamp and apparently a thread ID or something, some with a timestamp without a thread ID, some with thread ID without a timestamp, and some without any of those:
It would be good (and more informative) if all records contained a timestamp at least. mariabackup prefix might also be good, to distinguish the records from server-side ones when they are intermixed.
Server-side ones also vary, but that's out of the scope of this task. Here is a decent one, for example: