When gtid_strict_mode = ON an attempt to SET @@global.gtid_slave_pos to a value
not containing gtid domains from the slave's gtid_binlog_pos is reacted with an error, like

[test]> set @@global.gtid_slave_pos='0-1-2';

ERROR 1948 (HY000): Specified value for @@gtid_slave_pos contains no value for replication domain 99. This conflicts with the binary log which contains GTID 99-2-1. If MASTER_GTID_POS=CURRENT_POS is used, the binlog position will override the new value of @@gtid_slave_pos

The error message itself makes clear that the error is mere potential and there's no issue
when MASTER_GTID_POS = SLAVE_POS will be specified later.

The error issuing there should wait until MASTER_GTID_POS value gets known.