my granddaughter had this, and she had to have a colonoscopy and biopsies to dx why. they can check for nerves, cells of any kind, (hers was milk protein allergy) and advise appropriate measures to help the issue.
sometimes, for unknown reasons, a bowel will do this when its messed with too much. esp in gunshots, etc. it just doesnt ever wake up/ some may say the surgery had nothing to do with it, they may be right or not. a biopsy will tell. i certainly wouldnt pay much attention to that...they may not ever be able to prove why this happened. and again, a colonoscopy might show the reason.
keep searching. miralax was the only way my grand could go for many years. and constipation is truly dangerous if its bad enough.
good luck to ya !