I saw that happening on occasion with another game as well. I'm using the flib library to do a clamp, and calling that library's math.clamp(-0, 0, 100) returns -0 instead of a somewhat expected 0. However, mathematically, -0 == 0.
I agree it's ugly - I posted an issue to to flib -https://github.com/factoriolib/flib/issues/43 - waiting to see what that author comes back with. If it's a "won't fix" or takes a while, I'll fix it in UMB with a math.abs call on the next version bump.
