Currently the session scope variable is mutable within transaction which
probably never conceived to this way because is dangerous for replication.
In particular a transaction can be relay-logged any of its events removed including
Gtid and Xid events
which affects overall slave applier behavior, esp in the parallel mode.
E.g in master server_id=1 and slave server_id 2 the following transaction

begin; 

set @@server_id=1; insert into t set a=14; 

set @@server_id=2; 

commit /* to end up on slave in I */; 

ends up in the slave relay log as a singleton no-gtid event:

slave-relay-bin.000003 | 688 | Query             |         1 |        1193 | use `test`; insert into t set a=14