You may notice that after trimming a part of a boundary object enclosed in a space (for example, if you trim a part of a shaft,) the enclosed object is not recognized as a boundary object anymore, and the surrounding space flows into it.
When trimming a boundary object by substituting a new boundary object in the trimming position, the new object will not be recognized automatically as boundary object for the space, because it did not bound the space previously. In this case, you need to update the space manually to recognize the new bounding object. See the following example: 