use_sql_variables_in=master doesn't work

Setting the use_sql_variables_in=master parameter in the readwritesplit should make it so that all read queries which access a variable should be routed to the master. The queries are not routed to the master but are routed to a slave.

Steps to reproduce:
add use_sql_variables_in=master to a service which uses readwritesplit and execute the following queries.

set @myvar = @@hostname;

select @myvar; -- This should not return NULL

The following does work which shows that the select part of the query is the cause.

set @myvar = @@hostname;

begin;

select @myvar; -- @@hostname is returned