I noticed something weird while building a british BC (I think it was BC hull IV). After I had shrunk and expanded the displacement a few times, I was unable to place a barbette/gun on the upper deck any more. It kept trying to place itself at the level of the lower deck level, i.e. it was burying itself into the hull. The mounting points had gotten stuck in a configuration for a certain displacement, which didn't match the actual displacement as set by the slider. I tried fiddling around with displacement/beam/draught to get it to work but to no avail. But when I exited/restarted the builder it worked fine.
While doing this I noticed you could end up with some massive fore weight offsets. I think it's most prominent when you take the model all the way down to the minimum displacement and then up to the maximum displacement. I suspect that the center of balance might be also getting stuck in the wrong spot, and not corresponding to the displacement as set by the slider. Some previous tests with the placement of guns kind of suggested this. I might test this out further tonight, but yeah, something seems a little weird, especially when you expand out multideck hulls. I've never noticed any of this with single deck hulls.