test scenarion:

1. create one table, make first backup

sudo mariadb-backup -utest_user -p**** --backup --target-dir=backup/base_backup_dir

2. create second table, create next part of backup:
sudo mariadb-backup -utest_user -p**** --backup --incremental-basedir=backup/base_backup_dir --target-dir=backup/inc_backup_dir_2

repeat it several times

3. clean DB directory, start backup prepare:

sudo mariadb-backup --prepare --target-dir=backup_prepare/base_backup_dir
sudo mariadb-backup --prepare --target-dir=backup_prepare/base_backup_dir --incremental-dir=backup_prepare/inc_backup_dir_2
sudo mariadb-backup --prepare --target-dir=backup_prepare/base_backup_dir --incremental-dir=backup_prepare/inc_backup_dir_3

the last command fails:

sudo mariadb-backup --prepare --target-dir=backup_prepare/base_backup_dir --incremental-dir=backup_prepare/inc_backup_dir_3

mariadb-backup based on MariaDB server 10.4.26-MariaDB debian-linux-gnu (x86_64)

[00] 2022-10-03 11:43:22 incremental backup from 900277 is enabled.

[00] 2022-10-03 11:43:22 cd to /home/vagrant/backup_prepare/base_backup_dir/

[00] 2022-10-03 11:43:22 open files limit requested 0, set to 65536

[00] 2022-10-03 11:43:22 This target seems to be already prepared.

[00] 2022-10-03 11:43:22 error: This incremental backup seems not to be proper for the target. Check 'to_lsn' of the target and 'from_lsn' of the incremental.

full test log is attached (log104), files created by mariadb-backup - backup.tar.gz
the same test is ok for 10.5, 10.6, 10.7, 10.8, 10.9