For example, my default Kubuntu 19.10 installation flips the primary and secondary if the primary is unresponsive for a while. Since my laptop takes a moment to establish a WiFi connection upon waking up, it always decides that the primary server is down and to default to the secondary server. It has currently been 3½ hours since my laptop queried its primary server and it has queried the secondary server over 1000 times in the past 24 hours despite the primary having 100% uptime.

Most stub resolvers have an option to use strict order, but you can't rely on it as a network admin.