The real-world situation is this:
In the 1977 film, there was never any indication of a deliberate weakening of defenses. The rebels stole the plans in the hope of finding an accidental weakness, and found one.
Once Star Wars became the best film evah, and subject to endless analysis by people who analyze movies instead of getting laid, this sequence of events began to seem rather implausible. The builders of the Death Star were unable to find the flaw, despite decades of intimate familiarity with the plan, but the rebels were able to uncover it in just a few hours?
When the new film came out, the producers wanted to patch the problem: in the revised story line, the weakness (the unshielded "thermal exhaust port") was deliberately introduced to the design and deliberately disclosed to the rebels. None of this was mentioned in the previous movie, but neither was it contradicted.
There is still a problem, albeit a smaller one. The natural behavior of the saboteur, Galen Erso, would be to commit the location of the weakness (and whatever other information necessary to exploit it) to memory, or at worst, to some summary form, and pass that information along directly.
Of course, if the screenwriters allowed their character to do that, A New Hope would be retroactively made pointless, and they wouldn't have much of a story for Rogue One to boot. Instead, Erso carefully creates the weakness and only passes along enough information to tell the rebels what to look for in the plans, which the rebels need to obtain for themselves.
Had I been the screenwriter, I would have made Erso's plan to be the sensible one: to tell the rebels, "If you put an torpedo down the unshielded vent at 38.061927°N, 122.652542°W, the whole thing will blow up."
Then, of course, I'd kill off Erso before he could convey the details and give Jyn a line like "Alas, my father died before he could give us the exact location of the hole in the Imperial defense! Wait, the hole must be in there somewhere. If we analyze the plans carefully, we can find it. All we need now is to steal the plans."