No ER_SLAVE_HEARTBEAT_VALUE_OUT_OF_RANGE_MAX warning when setting @@GLOBAL.slave_net_timeout to below CHANGE MASTER’s master_heartbeat_period

CHANGE MASTER master_heartbeat_period TO greater than @@GLOBAL.slave_net_timeout gives a ER_SLAVE_HEARTBEAT_VALUE_OUT_OF_RANGE_MAX warning:

SET @@GLOBAL.slave_net_timeout = 20;

CHANGE MASTER TO master_heartbeat_period = 40.0;

Warnings:

Warning 1704    The requested value for the heartbeat period exceeds the value of `slave_net_timeout' seconds. A sensible value for the period should be less than the timeout

But not when the two configs are in the other order:

CHANGE MASTER TO master_heartbeat_period = 40.0;

SET @@GLOBAL.slave_net_timeout = 20;

A related phenomenon is that the default for master_heartbeat_period is computed from @@slave_net_timeout once the CHANGE MASTER entry is created.
Changing @@slave_net_timeout afterward does not update the defaulted config in existing CHANGE MASTER entries.