I've just installed 140mm on mine. Same problem. Looks like the mounts are piss, or maybe there's some paint that's underneath causing the caliper to tip over when tightened.
I had to have my flat mont fork faced to get a good it. Might me paint as you suggest.
@motoko started
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I've just installed 140mm on mine. Same problem. Looks like the mounts are piss, or maybe there's some paint that's underneath causing the caliper to tip over when tightened.