Problem with Alexa Groups

I have a Sonos Beam and an Alexa dot in a group called ‘kitchen’ i have set the preferred speaker as the beam but when I say ‘play music’ and the dot picks it up (as it invariably does, it seems to be more sensitive hence the reason i have added it to the kitchen set up) it will play through the dot not the beam unless I specify ‘play on kitchen sonos.’ My understanding was that the preferred speaker option was supposed to prevent this. Any help appreciated.