We didnt notice it at first we had a giant 90' pine fall into tree and a portion landed in step area. We thought it was a scratch at first and it got bigger/ A little dye confimed a leak. My pool guy put underwater epoxy but it looks like crack is expanding. My idea would be pool drained below stairs, holes drilled to stop the crack. Then they make expandable epoxy filler.
All the quick fixes are just really bandaid they wont hold permanently. You can glass them in or bondo amd scuff the whole thing amd gel coat. The last ones I repaired were beautifully done. We drilled and filled with non shrink grout then fiberglassed in resin and 2 part filler amd recreated the step pattern with a dremel. My one guy is an artsy type he did a nice job. Then a local ecofinish guy shot the stairs in grey ended up awesome they were white. He also did the skimmer lid the stair mouldings and skimmer face plates changed whole look of pool for the better.
I used 3M Marine filler to fix the crack and painted with Gel. Assuming you have a firm step?
otherwise, you have to cut a hole to fill in the voids below the tread. I used a kit from :Home - Pool Step Specialists
Attaching photo of finished steps, before getting new liner installed. I patched 7 holes and blisters.