copy-back is not restoring binlog file properly when absolute path in used in log-bin

my.cnf include the following information about：

[[email protected] data]# cat /etc/my.cnf |grep 'log-bin|datadir'
datadir = /data/mysql
log-bin = /data/mysql/mybinlog
1.do backup

2.do prepare
prepare is ok ， and I get the following values like :

[[email protected] backup]# cat xtra_full_20210515160208/mybinlog.index
/data/mysql/mybinlog.000005
copy-back
xtrabackup --defaults-file=/etc/my.cnf --copy-back --target-dir=/data/backup/xtra_full_20210515160208/
When I finished executing --copy-back, I found that there was a problem,The index file of binlog is missing some information，When I change it to mybinlog.000005，MySQL can be started normally。

[[email protected] ~]# cat /data/mysql/mybinlog.index
/data/mysql/.000005

https://forums.percona.com/t/is-this-a-bug-of-xtrabackup-8-0-23-16-and-mysql-8-0-23-when-i-executed-copy-back-there-was-an-error-in-the-index-file-of-binlog/10472