When using the transform panel to resize an object or multiple objects, it goes without saying if you select constrain, there should be confidence it will scale respectively. A circle remains a circle, a square a square, a 16:9 Graphic stays 16:9 etc. etc.

Unfortunately I have noticed a very concerning bug where if one of the objects has a stroke, it will not transform constrained. I noticed because the object I grabbed had a perfect circle, that seems maybe 5% horizontally compressed afterward.

I have done this screencast to show the bug does not occur on non-stroked objects- only on transforms applied to ones with strokes.

I have no idea how long this has been the case, but I have done the screencast on a new document to replicate the findings in hope a patch can be done asap.

I also have AI CC 2018, so will perform the same check also and post if not resolved.

Here is the AI CC 2017 screencast https://youtu.be/q0YK-OOdA5s

Here is the AI CC 2018 screencast https://youtu.be/NFW54mMVyDg