Bret Devereaux

...of Livy and Polybius are lost, and all we have are later summaries of their work. Often that means literally knowing, "An important battle at <place> happened in this year and <side> won." Sometimes we're left trying to sketch out the events of a decade from...

...really disjointed biographies (esp. Plutarch) because while there *were* narrative sources for these important battles, they don't survive. And that's with a fairly complete reckoning of all of the material that made it out of the classical past.

Faramir's situation is worse: maybe the information exists, but it's in Rivendell (a long way away) and it may be in a language that few in Gondor speak. Getting that information means sending someone all the way to Rivendell - hardly a safe journey, as we see!

And then probably getting it copied down into text, and getting that back to Minas Tirith for the production of further copies. Hard in a world as disconnected as Middle Earth, where the knowledge of the Dunedain in the North is mostly cut off from the south.

...environment, their having it and even using it doesn't percolate out to a general knowledge and availability of the source until much later. In that context, I wouldn't be surprised that for Gondor - which after all has had bigger problems to worry about...

...that keeping the exact history of the northern kingdom straight just wasn't the top priority. And by the time you realize the information chain is broken, it's too late to repair. In Unfinished Tales, even Saruman and Sauron struggle to get good information on Gladden.

I find that I am fully in concurrence with your point in general terms (there would be a great deal of ignorance of the history on either side, not withstanding mission of Earnur et al), while still struggling to believe in reference to Isildur specifically.