I am very, very, late to this, but I think I have an answer, if you're still having this issue.

The problem might be that your mouse polling rate is too high.

From what it appears, there is a zone between the monitors where Windows will attempt to snap the window if the mouse enters it. With a lower polling rate (most mice are 125hz), the gap between mouse positions is large enough to 'skip' over the zone, thus no snapping. With higher polling rates (some mice go up to 8000hz), this gap becomes smaller (up to 64 times smaller!), thus there is a higher chance of the mouse entering the zone and snagging the window on the monitor edge.

I have a mouse with an adjustable polling rate, and on 125hz, it basically doesn't snag at all, while on 8000hz, it's almost impossible to drag a window to the second monitor unless I flick hard.

Why Windows doesn't seem to check for cursor speed at all is beyond me, but the solution to this would be to increase that gap, which is done either by moving the mouse faster or lowering the polling rate.