Tile Grout: Patch up gaps, or strip out grout and regrout the whole thing?
You could push some grout into the bare areas but even if it was original from the same bag it will now be a different shade. I usually strip the grout for a level but not the entire wall I might cut just a slight amount on everything and when filling the bare areas work everything so the color matches or is not as obvious. After 16 years I probably would not be looking for flexing in this case and a refill will probably be fine.