Death Star engineers: smart enough to design the ultimate space weapon, but not smart enough to correct design flaws/security measures once they knew the plans were stolen.
Maybe the problem was still in committee.
The time frame from the theft of the plans to the assault on Yavin IV was only a few days, and I think everyone on the Imperial side who knew there was a fatal flaw ended up getting killed.
It's like two days or something, from A New Hope's start to its finish. Unless the Sandcrawler takes a heap of time from picking up the droids to arriving at Owen's place. Anywho, it's brisk.
In ROGUE ONE, it's eventually clear that Scarif has a security problem, and once Krennic arrives it's known that data has been sent from the Imperial base, but it's not inherently clear if the Imperials know what data was stolen.
Even if they /do/ find that it's the Death Star's plans, it's possible that the Imperials want to show it to any planets that are wavering on aligning with the Rebels, pushing them to join.
Imperials are so overconfident with their new toy, it's likely they don't suspect that there is a flaw. I mean, Krennic does, but Erso was "his hire," so he's not going to advertise that with Tarkin trying to push his shit in all the time.