mariadb-backup skips DBs matching the --databases pattern

When trying to run a partial backup with a pattern exactly as described in the official documentation, the mariadb-backup tool does not actually match the given pattern and skips databases.

Here is a small reproduction to server as a basis:

CREATE DATABASE IF NOT EXISTS app1_first;

CREATE DATABASE IF NOT EXISTS app1_second;

(you can also add tables and data into these, which was obviously the case for my production environment, but the issue is reproducible regardless of whether data is actually present).

Once these databases exist, run the command as described in the linked docs (this is slightly adapted but syntactically the same):

$  sudo mariadb-backup --backup --target-dir=/home/myuser/mariadb_backup_test/ --databases='app1_*' --user=my_db_user -p

In the backup_logs.txt file, you can see multiple lines stating that databases which clearly match a pattern like app1_*

Check the mariadb_backup_test directory and see that there are no folders for any of the app1_* tables.
Because this worried me, I then ran the backup prepare command, thinking that maybe those folders only appeared once a backup had been prepared.

$ sudo mariadb-backup --prepare --target-dir=/home/myuser/mariadb_backup_test/

You can find the full logs attached as backup_prepare_logs-2.txt.

Why is this important?

Firstly, I believe a feature described in the documentation with its own full page should work as described there.

Secondly, I would like to use partial backups because I have a multi-tenancy setup where I may wish to backup only specific tables to specific directories or similar workflows.

—
I was directed here from my original topic on the Zulip chat.
If further information is required, I will do my best to provide it.