Bed not staying level

I bumped up against this same issue. Figured out it was the line in the startup code right before the UBL line that was changing the bed temps.

Anyone with even a brief understanding of thermo-dynamics knows that when you change the temperature of a material it changes in size (expands as it warms/contracts as it cools).

So here in the startup gcode is a call to set the bed to 60C no matter what is was before.

M140 S60 ;non blocking heatup of bed
G28 ; home all axis
G29 ; probe for bed autolevel plane

During the UBL routine the bed is constantly changing temperature and of course things are going to move until not only the bed temp has stabilized but the bed support arms have stabilized also.

I discovered this while "trying" to print ABS with an extruder temp of 240C a bed temp of 100C and an enclosure stabilized at 90F.

Just before the UBL...the M140 S60 started dropping the temp of the bed and the angle started changing enough to throw off the UBL measurements.

You can verify this this using a digital level (with a readout in 100ths of a degree) placed fore and aft on the bed and watch the bed level change. And yes...the digital level was stabilized at the original bed temp and raised above the bed on shims at the rear and forward end of the bed so as to reduce thermal changes in the level itself as the bed changed temp.

My suggestion: 1) Remove that M140 S60 line of code. 2) No matter what material you are printing...preheat (if necessary) your bed and enclosure until both stabilize...then start your print. So far this discovery has fixed in part the problems I was having trying to print using ABS.