A number of years ago when i was printing on a thick aluminum plate 12mm cast fixture plate i was having the same issue using bang bang for the bed heater. If you put a dial indictor on the place you will actually see the dial indicator moving up to .2mm during heating. I changed the bed heater to PID and the issue was greatly reduced. One issue with aluminum is the thermal expansion which is why i have gone to borosilicate glass and all the expansion problems went away.
