I realize now you have a 12x12 bed. With more current flowing around for the large heat bed, any issues with the current distribution would be more pronounced, perhaps as you are seeing. Maybe the design *is* bad, or maybe there's a problem in manufacturing inconsistency.

I'm going to be revamping my bed and print surface within the next few weeks - now I know to put a bit of scrutiny on how the traces were routed on my 8x8 bed. I've been holding off on my bed revamp for a long time, hoping that Panucatt would eventually restock their Helios bed, which sounds like a good design. I really liked the idea of being able to eliminate the screw heads sticking up in each corner. After waiting a year with no response from Panucatt I'm giving up on it and going in another direction reusing the MakerFarm MK1 heat bed.

It's hard to fathom that the original concept behind the Prusa bed was to just print directly on the back side of the circuit board. At least I remember reading that somewhere. The idea to use glass came later when people were having issues with the circuit board flexing. Duh.