That grain is setup perfectly to crack. Could try and reinforce with some screws or dowels
Yeah, that's pretty much destined to crack.
That's a middle step in the photo, and it looks OK, but what to do about it the bottom step depends on what, exactly, is creating the greater stress.
If it's the fact that it's closer to the ground, and thus weathers and deteriorates faster, then that's one thing. Or maybe it's getting damaged when mowing, etc. Maybe use a plastic composite instead of wood.
If it's further from the ground or sidewalk than the spacing between the rest of the treads, then maybe it's causing people to break stride a little too much, and step on the "nose" instead of the middle of the tread. Changing the spacing might help. Of course, that's also potentially a major project of replacing the entire set of stairs.
It might be easier to try and eliminate the overhang, perhaps with wedge-shaped reinforcements screwed to the stringer. I could also envision simply installing a plank to span between the stringers to support the overhang.
Or, you could install a metal "stair nosing" or "sill nosing" strip to reinforce this area from the top, if it looks more like people are stumbling into it, or the nose is getting damaged.