I suppose it might be theoretically possible for a zoom lens design to have a shorter MFD at a shorter focal length, but I've never came across such a lens.

Most zoom lenses have an MFD that is either the same or very near the same for all focal lengths. This necessarily means the magnification will increase as the focal length increases given that the MFD is the same.

A lens with a retrofocus type of design such as is used for wide angle lenses would be my guess as to the most likely candidate to be the exception if some other element of the lens' design, such as internal focus and/or internal zoom, prevented movement of the lens' elements to allow for the same MFD at longer focal lengths as the MFD allowed at a shorter focal length.

The vast majority of telephoto zoom lenses have relatively small MM: somewhere in the neighborhood of between 0.15X and 0.20X. Some may approach 0.30X, but they are the exception more than the rule. They accomplish this MM at the longest focal length of which the lens is capable.

Most zoom lenses in the normal range of about 24-70mm have MM that ranges from around 0.20X to 0.30X. There are a few exceptions of zoom lenses with significantly higher MM. The Canon EF 24-70mm f/4 IS has a MFD of around 8 inches which gives it an MM of 0.70X. This magnification is only achievable when the zoom ring is moved past 70mm to the Macro position, and infinity focus is lost at this setting. When set to 70mm the MM is a fairly pedestrian MM of 0.21X at about 15 inches MFD. This is similar to most other 24-70mm lenses made by Canon, Nikon, Tamron and Sigma.

For true 1:1 macro capability, the available lenses are almost exclusively prime lenses. This allows lensmakers to avoid both the complexity of a zoom lens and the extension needed by a macro lens to focus on short distances to be combined in the same lens design.

The Canon MP-E 65mm 1-5X Macro is a one-of-a kind lens that some might define as a zoom lens, since the field of view changes by a factor of 500% from 1X to 5X, that is only capable of a single focus distance (e.g. the MFD) at each zoom position. Since focal length is defined by how much a lens bends collimated light to focus it at a particular distance, and since the MP-E 65mm 1-5X can't focus collimated light at the registration distance for which it is designed, in a sense it has no truly definable focal length in the conventional sense.