How to fix plywood subfloor butt joint that is off by 1/8" before installing hardwood flooring
Use a large hand plane like this:



I installed vinyl plank throughout my first floor; 1200 square feet.  The rooms I did before thinking about this show every little bump or seam.  Most people don't see the defects, but I sure do.  They drive me nuts.  I wish I thought of this sooner.

Make sure the blade is set to take tiny ribbons of wood and is real sharp.  It will go through the wood like butter.  Makes quick work and you can get that area completely smooth.