Why do seemingly unrelated things affect rendering?

I’m using Flamingo, and am thoroughly enjoying learning it. However, I frequently find myself asking why things, that are seemingly unrelated to rendering, have a dramatic effect on the outcome?

For example, if I change from 300k to 600k max pixels, my exterior image looks a lot better, but…all of the interior lights appear to be turned off and the windows are dark? The same thing will happen sometimes if I change one pattern—such as in image used in a water material—with another.

Or, when I’m using a grey-scale displacement bitmap, it works just fine until I edit the bitmap, and reload it, to add a few more bumps. Then, it appears not to work at all, and the material to be displaced suddenly looks perfectly smooth, instead.

If it would be helpful, I can upload the most recent problem file, but it’s almost 50Mb. Also, could these types of problems be related to the use of many lights? In this latest file, I’m using close nearly 50 interior/exterior lights.