Some viewers in the Dallas Fort Worth area have reported issues regarding KXAS NBC 5. This may not be related but who knows? It is inconsistent.
This is a stooopud question but after a rescan did you check to see if there’s a 
check beside 5.1 in Channel Lineup?
Mine has turned on and off random channels without me touching them. I think it’s caused by stations dropping off then back on at some time that’s relevant.