This was indoor in dim lighting conditions. The increase was ~20% or so at every level.
Again, this is working by design.
I tested an 11 pro, 12 pro, 12 pro max, and 12 mini side by side in the same room at 30% 50%, 90%, 100% brightness and they ALL increased in brightness at EVERY level when AB was turned off.
The issue with sourcing your 'testing' from multiple people is that you have no constant.
Every test is going to have different lighting conditions with people who may or may not notice the change, or interpret your testing the same way.
Testing in the store or any bright environment is again not a valid test.
When you are in a bright environment the ambient light sensor will max out the brightness range.
In that use case, there will be no brightness change with AB on or off, even at 100%.