Not a big fan of pore filler on stuff. but might I suggest trying the technique out on a piece of walnut scrap to see if you like it before committing your piece to it.
End grain? or just more open on the ends of a flat sawn board?
I have never done it to end grain, don't know what would happen.
On regular long grain, the water-based stuff I used worked well.
That's what I expected. I would have no problem with filling it in that case, because just sanding more won't accomplish anything. As you sand away some pores, you open up more new ones. It's a dog chasing it's own tail
I have used timbermate's Walnut wood filler as a pore filler in some select spots. Sometimes I add a little bit of red oak and mix it together to lighten the color. If using as a pore filler, I think you'd be okay either way.