I just found this comment in a post I made a month ago:
"you can defiantly see the it flexing in the middles on the keyboard and palm sides of the trackpad. I thought it was just mine but I experienced the same flexing at both Apple & Best Buy on their displays."
So, I think the fexing is normal. I can get almost 1mm when doing the 2nd click press if I click in the middle of the trackpad near the edge. I don't know if others can confirm this, maybe people have different situations but I never even noticed it before. But it seems normal to me - the Force Touch works a bit differently than 3D Touch on the iPhone and Apple Watch - there you have capacitive sensors that measure micro changes anywhere on screen while Force Touch seems to have physical pressure detectors that require the surface to move to actually move.
I think this is quite normal for trackpads of this size - the 2016 15" Trackpad and Magic Trackpad 2 have the same width and they flex at the middle.
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Don't have that issue, but I don't rest my palms on it while typing. A lot of people type with wrists down, but it's a bad posture. Still, if you're having this issue, disabling tap to click may help.
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Are you saying yours doesn't move when you press it hard at that spot? Middle of the trackpad, just below the edge.