Yes—on some constant-aperture zooms, this can be intentional rather than a defect. Some designs use a secondary diaphragm or other internal mechanism that changes as you zoom to keep the effective maximum aperture constant across the zoom range. Canon, for example, has used a secondary diaphragm in some wide-to-standard constant-aperture zooms, and its movement can be visible while zooming.

This is done because zooming changes focal length, which affects the effective f-number. To maintain a constant maximum aperture, the lens may need to mechanically adjust an internal stop or optical geometry.

That said, not every constant-aperture zoom does this in a visible way. Some lenses show it clearly, some only slightly, and some telephoto zooms may not use the same approach at all.

So if a lens appears to stop down somewhat at certain zoom settings, that is not automatically a fault. It may be normal for that lens design. A problem is more likely if the movement is irregular, sticky, inconsistent, or prevents the lens from opening or stopping down properly.