To reproduce:
1. Open attached project
2. Open "AspectRatioFitterBug" scene
3. Play the scene
4. Set aspect ratio to 4:3
5. Set aspect ratio to 16:9

Actual behavior: left button will remain square as expected, because of the script it has attached. The other will become rectangular
Expected behavior: square in the right should behave in the same way as the other

Workaround is the script attached with these lines of code:
float restore = fitter.aspectRatio;
fitter.aspectRatio = fitter.aspectRatio*1.001f;
fitter.aspectRatio = restore;

Not reproducible in 2018.3.0a1