Yep, I was able to reproduce the bug/feature immediately in a new scene with just a plane and a Forest object on top of it. This all happens when using a brush of a size, e.g. 200cm, and then painting with a size of 100000cm. It is definitely the same reason it happened in my actual scene I'm working on, because I vary the size of the brush a lot - we're talking from 300cm to a thousand times that or so. Obviously, this can be pretty problematic if the user isn't aware of this until it's too late. Fortunately, I keep a lot of iterative backups so I was able to salvage most of my work.
Now, am I doomed to having to use multiple paint areas for different brush sizes, or can this threshold be modified? I'm creating a really huge scene that calls for more detail in some places, and some areas are just vast forests, so I need to vary the brush size this drastically. In fact, I find I can't change from 200cm to 2000cm without the resolution of the paint area being compressed. Is there any way to turn this mechanism off? Otherwise I guess I'll have to stick to always doing the larger areas first and then only decreasing the brush size as I go.