Compiling with ASAN, one gets memory leaks with:
mtr mariabackup.error_during_copyback

Direct leak of 8248 byte(s) in 1 object(s) allocated from:
#0 0x7f04b50dc110 in malloc (/usr/lib64/libasan.so.4+0xdc110)
#1 0x6f6ae5 in datadir_iter_new /my/maria-10.6/extra/mariabackup/backup_copy.cc:206

Direct leak of 512 byte(s) in 1 object(s) allocated from:
#0 0x7f04b50dc110 in malloc (/usr/lib64/libasan.so.4+0xdc110)
#1 0x6f6f5a in datadir_node_fill /my/maria-10.6/extra/mariabackup/backup_copy.cc:167

This could be related (also with ASAN)
mtr mariabackup.datadirectory also fails with:

sh: line 1: 13898 Aborted /home/my/maria-10.10/extra/mariabackup
/mariabackup --defaults-file=/home/my/maria-10.10/mysql-test/var/14/my.cnf --bac
kup --target-dir=/home/my/maria-10.10/mysql-test/var/14/tmp/backup 2>&1
mysqltest: At line 32: command "$XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my
.cnf --backup --target-dir=$targetdir" failed with wrong error: 134