In a few odd models, sometimes "something" breaks and some skins don't change when you select a new appearance from the skin menu. Three models that exhibit this are skeletalwarhorse.m2, sharkray.m2 and hippocampusmount.m2. The issue doesn't always happen on the first two models (but always does on hippocampusmount), but if you click back and forth on this model and another it will eventually occur. It only affects one skin per model (the 2nd of skeletalwarhorse or the first of the other two), so the remaining skins will change, while the geosets that use the broken one take on the appearance of one of the other skins.
For hippocampusmount.m2 there are 2 skin sets: when you select a new appearance the body colour won't change, but the saddle does. For sharkray.m2 there's only one skin set and it always displays just the first one when it breaks. For skeletalwarhorse.m2 there are 2 skin sets: the body will change but the saddle/armour breaks and takes on the same appearance as the body skin.
Usually you can fix the issue eventually by clicking back and forth on another item in the file tree (but not for hippocampusmount, which remains broken).
skeletalwarhorse comparison, normal vs broken: