That's the expected behavior, if you want to use it with just one they have a solution for you:

> To use Active Noise Cancellation with one AirPod only, use your iPhone or iPad to go to Settings > Accessibility > AirPods, and turn on "Noise Cancellation with One AirPod." Then press and hold the force sensor to switch between noise-control modes.

https://support.apple.com/en-us/108918

That's an intentional setting (presumably because it wouldn't really make sense?) and can be changed in AirPods settings