I have had similar issues, but I think that I might have overcome the problem by doing the following:
Clean off any filament residue from the end of the nozzle.
When you start the levelling sequence, tighten up the finger screw at the back a few turns and do the same at the front of the bed. Do that before you start winding up the bed with the thumb wheel. This puts a bit more tension into the springs.
Make sure you adjust as carefully at the 1mm stage for all three points as you do for the paper 'fine' gapping. (I use a piece of card that is about 1mm thick.
In the final two thumb screw adjustments I think it's quite important to be really careful to match the left and right clearance....yes, I agree with you, it's really easy to get the bed to flex downwards when you push the paper in the gap, but a sensitive touch is the important thing.
I am new to the UM2 myself, but still, I hope I have been some help to you.
That's a weird one. Maybe check if all the rods are straight?
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