It looks like a single joist at the bottom of a stairway is about 1/4-3/8
inch higher than the rest of the floor resulting in something of a little
hill.

It appears that simple sanding didn't work before, so better be brave and redo
it right. Given that problem, I'd examine the subfloor, remove a substantial
part of the plywood sufficient to access the offending joist(s) and get that
joist level before replacing the plywood. Sistering the bad joist might be a
good deal especially in front of a stair, because that is the last place you'd
want a spongy step. All this might seem like a lot of work, but an old timer
would have it trimmed out in a couple of hours. Good luck.

Joe