It looks like you used some lead filler to cover up the rust and rust remover.
JMHO, but I would put something like a spray on/paint on pickup bed liner such as Rhino liner ...etc. in that area to further protect from abrasion. It looks like a combination of poor drainage and some abrasion caused that problem. If you did use the lead it will not last very long.
Logic is a systematic method of coming to the wrong conclusion with confidence.
