region preference is just one of many matchmaker parameters for its decision of the player’s server.
This parameter has a weight along with other parameters such as number of gamers on the servers from the same region, their tiers, etc…
So matchmaker assigns just a one total “weight value” for each server from every region and then choose the one that has the higher weight.
So you can find yourself being dropped at RU or EU server instead of your preferred “US”
To know wich server you’v been dropped at open your game.log and search the latest string with “client: connected to”