Much like the Gov't, they invent cures for which there were no known diseases. By making one change in order to solve one problem, they end up creating 3 other problems (i.e. the fix made more problems, not less). The WORST thing about the 'fix' was that it took all the glory away from the IV team who found the weakness. So, now, they really didn't do anything special. (Even if her dad already told her what the weakness was and they didn't need the actual plans.) In IV, everyone had a heroic role to play but in R1 they're portrayed as a bunch of bumbling idiots with nothing to contribute.
Let's face it, some human beings just can't help themselves when it comes to messing with the way things are. They're always adding in little tidbits where they are not desired, expanding things until there's nothing left to expand. At which point, they make up new stuff. Some people really like that. I am not one of them.
I completely disagree. Galen told Jyn there was a lethal flaw in the plans but he didn't have time to tell her exactly what it was (it was cut short by Jedha City exploding i think), so the analyst team in Episode IV still did a good job in finding the one flaw in the plans, which must have been a huge amount of data to sort through. And they came up with a battle plan on the spot, which is not bad.
Actually he had plenty of time to tell her that super important bit of information, that he devoted years of his life to. He could've you know, not spent 95% of the holorecording talking about his daughter. "Hey! To whomever get's this message, get it to the Rebellion! Tell them I built a flaw into the design of this thing, it's in the exhaust ports! The full specs are in this data place on this planet (i can't be arsed to remember the name of it, most of the movie has faded from memory for me). But yeah, even if you can't find the plans, just look for the exhaust ports along the main diameter of the station! Shoot it with a torpedo or something and you'll blow it all up! Oh yeah, and Jyn, I love you, I never stopped loving you. Not that I have ANY way of knowing if you are alive, or how in the hell someone would find one person in an entire galaxy, who has been spending most of her life off grid, avoiding notice as much as she can. But yeah, I have faith this will find you, so I love you kid. "
Boom, information conveyed And he even had time to give her a message too.
Except the design flaw wasn't in the exhaust ports. It was in the reactor module. Basically, he said if you make a big enough boom at this spot, the entire thing will blow. But he didn't know exactly how the Rebellion would be able to exploit the weakness. Maybe snubfighters. Maybe a saboteur. Like a good GM, he told the party what to do, not how to do it.