When I open an image file, in this case a 16bit tif, and the select "Colors>Levels" the image is changed without my having done anything.  Doing the same thing on GIMP 2.9.8 worked as expected (i.e., did not do this).  Interestingly the left most value on the "Input Levels" (? black point) has a value different than 0 on GIMP 2.10.0-RC1 whereas doing the same thing with the same image file leaves it as 0 (zero) on GIMP 2.9.8.

I don't think GIMP should be altering the image without me requesting it to do something.  In this case it does seem that canceling the "Levels" dialogue appears to undo these alterations.