In the function DetachReplicaOperation, it run the command change master to to detach a lost replica,
like change master to master_log_file='//mysql-bin.000001:4788', master_log_pos=4788

On the command line, I simulated this scene, and an error occurred：

mysql > stop slave
mysql >change master to master_log_file='//mysql-bin.000001:4788', master_log_pos=4788;
ERROR 1776 (HY000): Parameters MASTER_LOG_FILE, MASTER_LOG_POS, RELAY_LOG_FILE and RELAY_LOG_POS cannot be set when MASTER_AUTO_POSITION is active.

context:

MySQL 5.7.21 with gtid

Please check it.

@MOON-CLJ