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I've spent a ton of time wrestling with this issue, and I don't think it's related to Windows Snap features. It's possible to have your monitors aligned in display settings where they APPEAR to be perfectly adjacent, but actually aren't - there's a pixel or two of dead space between them. You can use the arrow keeps to move the monitors around (in the display settings screen) which sometimes helps. I just discovered that if you hold shift when pressing the arrow keys, it seems to move the monitors in very small (single pixel?) increments, which finally did the trick.
Also, when using the arrow keys to relocate your monitors, one monitor can 'push' another monitor, but it can't push two, so if you have three monitors in a horizontal row, you should try pushing the outside monitors towards the center one.
Attached is an image with examples. Top is my setup, 2nd one is after I moved monitor 1 with Shift + Right Arrow, and bottom is after I moved monitor 1 with Right Arrow (no shift).
Hope it helps!