WiFi continually beacons out to try and find previously connected networks and will select for the best signal from an AP it can reach. Extenders can be a trick if you’re sitting in the ‘crossover’ space between the extender and the back haul it connects to.

What you might try instead is one of those distributed AP systems like Unifi or similar where all the APs are controlled by a switch and work in unison. The one I have at least has an ability to disconnect someone if they drop below a certain level and migrate them to another AP without breaking the session states.

The other option that I can think of is just turning off the auto connect for the extender net and only using that manually.