I contacted ISM (I have a ISM PN 3.0) and they mentioned that this usually happens when a saddle is too high. Indeed, I remembered raising my saddle a bit a few weeks ago. I'll lower it and see for my next metric century and who knows, if it goes well, it might turn into a real century

Makes sense.. to have such a hole down the middle they have to have a wider nose,

essentially 2 of them, side by side..

I think there is some truth to post #3 , in that where you are sitting is wider tan you have space for. But it may not be altogether due to a wider saddle. There would be the same effect of too much width if the saddle were tipped a bit back so that you slid aft just a bit. I have a saddle with a cut out also. If it begins to cause discomfort, I either slid forward or drop the nose of the saddle a touch.

The ISM looks like it has a wider nose than other seats. In fact, the ...prongs? horns? pieces that stick out toward the front? ... are a lot shorter than every nose I've seen. So the comparison isn;t the nose of other saddles. It's the taper of other saddles back where a normal saddle starts to taper towards the nose.

I tried one and thought it excruciatingly painful due to the width of the nose. My bod's not shaped like that - my legs are too close together. OTOH my nominal saddle width is 155, so I don't have a narrow butt.

Too much time in the saddle, not enough time out of the saddle. I once experienced bruised sit bones while tandeming without getting off for long periods.

I'll try to remember to alternate between riding on and off the saddle next time I go for over 3 hours riding.