In the Relay.ini file or module configuration for Relay there is a "Reselection" value. By default it is set to 0. If you set a value, then clients attached to a relay will go through the selection process to connect to the closest relay.
RELAY.ini = Default
; This parameter defines a timeout after which a new parent may be elected. This
; new election may only be triggered if a parent is currently connected. Its aim
; is to provide the capability to find a better relay whenever possible. Empty or
; 0 value will disable the reselection process.
The value is also available by using the BCM console.
Be aware in earlier Version the default value wasn't "0". It was "3600". Not sure if this was changed with 12.0 or before.