A master hides user name etc. somewhere in the binlog while writing a record, so that a slave can later use them for CURRENT_USER and such.
They can be seen in mysqlbinlog --hexdump format if you know what you are looking for; but normal mysqlbinlog output does not show them. Maybe after fixing MDEV-8628 they won't be hidden any longer at all, but if they are (or if MDEV-8628 is not fixed any time soon), it would be really useful at least to see them.