MdlVis can bug up with geoset visibility sometimes. When it doesn't work in MdlVis, use War3ModelEditor to change geoset visibility. Do it by creating a geoset animation or by editing an existing geoset animation of that geoset whose visibility you want to change in Geoset Animation Manager. Tick the "Animated" in "Alpha" in the wanted geoset animation if it isn't already ticked and click the button "Alpha". In that window, enter the number of the first frame of the animation you want to change visibility. To get the first frame go to Sequence Manager and look whats written under "From:" when you double-click on the wanted animation. After that number in the geoset animation "Alpha" window you should put a colon ( : ), then one space, and then number 0 if you want the geoset to be invisible in that animation or number 1 if you want it to be visible, if it wasn't visible previously. Note that geosets are visible by default, and you don't have to enter those with the number 1. This way, you can even hide the geoset in question in the middle of a certain animation by entering a frame number that's in-between the numerical values of the starting and the ending keyframes ("From:" and "To:").
Note: If you can't find the geoset you need to assign the geoset animation to, Ctrl+Left click it in the model preview window (on the model) with the Geoset Manager open and it will be highlighted there.