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As a general tip, in OpenSCAD the bodies that are subtracted from other bodies should always extend beyond the original object where open faces are desired.

The reason for this is that in digital computing, floating point numbers are of finite accuracy, so subtracting something of "equal" size, which would be fine in theory, doesn't work in practice, because due to the inevitable inaccuracies, some tiny bits of object always fail to be removed properly.

You might have noticed that your previews look "untidy" in the open faces ... that's the result of this effect.

Just make the sutracted body 0.01 mm higher than theoretically required, and all will look fine and work reliably.