Hello.

If you change the resolution but keep the same aspect ratio, for example 1920x1080 to 1600x900 or 1920x1080 to 1280x720, is sensitivity or muscle memory affected?

Short answer, no... in most cases*. 360 distance, FOV and sensitivity will be the same. If there is a change in monitor size, it might be perceived differently (even though it is technically the same), which you can account for in the calculator by entering different monitor sizes.

*A few games are directly affected by resolution (in most, if not all, cases horizontal), for instance
Farming Simulator: https://www.mouse-sensitivity.com/?share=b5c6f73895ada04db4dddb6e81756a95
and
Minecraft Bedrock Edition: https://www.mouse-sensitivity.com/?share=cee3b246ffafe18232655f2a025b8d75

These are rare though, and the calculator will account for this unless it somehow didn't get detected under analysis.