Weird Makergear M2 Heated Build Plate problem

[UnknownTarget]

Hey everyone. So this Makergear M2 I've got prints fine, except the temperature graph consistantly reads zero once the build plate gets about 3/4 of the way up the Z rail. I looked for any obvious signs of shorting - wires moving around, etc, but nothing pops out. Any ideas?

Note that it does read temperature properly when printing (IIRC) and when it's below 3/4 of the way down.