A lunch bag that smells is almost never dirty on the surface people can see. It is damp somewhere they cannot.

The stitch line is a perforation

Sewing a liner in punches hundreds of holes through the waterproof layer.

Liquid finds them on the first spill and goes behind the liner.

The seam allowance is absorbent cloth

Behind the stitch line sits raw fabric, folded, in a sealed pocket.

No cloth, sponge or brush reaches it, and it never fully dries.

Which is why wiping does not fix it

You are cleaning the one surface that was never the problem.

The bag looks spotless and still smells, so people assume it is just old.

The fix is a build decision

A welded liner has fused seams: one continuous surface, no allowance, nothing hidden.

Where welding does not suit a format, bind the seam so no raw edge is exposed.