Something wrong when using " union "

Hi, Everyone

I am trying to draw an geometry, as shown below, using the button “union”. At start, there is one hopper and one cylinder used to be the outlet of the hopper. It is expected that the united geometry has the external contour of two original geometries. However, the result is out of the expectation. The outlet is inside of the hopper.

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Before writing this poster, i made a test to validate the function of “union”, and the result shown below, is consistent with my expectation.

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Both of source files of two cases are uploaded.
test.mfx (5.7 KB)
pressure_relief.mfx (13.3 KB)

Can you help me to solve this problem? Thank you!

Best regards.
wuming