After playing around with it a bit more I can definitely reproduce the issue every single time if one uses the Layer -> Transform command to scale the bump bitmap. The resulting bump map effect will look broken at higher resolution. Using the same exact procedure and rendering at 1/3rd of the resolution will look great though.
For the above I'm specifically comparing 6k vs 2k images.
All that said, even if I render a single map in 6k and compare it to 2k there are differences in the way bump is applied. It is less pronounced but it is there and it can look quite a bit different. I've tested it with and without CBitmaps but ultimately this particular point probably comes down to texture filtering that was already reported before.
So from my quick re-testing here it seems like yeah, we currently don't have the ability to scale bump maps in Corona for Cinema 4D - the transform is buggy (as noted in the first paragraph) and the projector doesn't work / isn't supported. This is a major problem for production work :\