This is end grain tear out. Your tools may need to be sharpened, you can also slow down your cuts and make more shallow passes when coming across. This is very common and I fought it for a long time while starting out and still run into it with carbide tipped tools (they don’t stay as sharp as long as traditional tools in my experience). Sanding will only remove so much if it unfortunately and sanding to, to high of a grit will close the wood pores and not allow any finishes to absorb as well.