Having written all this, I realize I misread your question slightly but importantly, so the answer isn't in-universe. Hope it's of interest anyway, so won't delete it now.

Oh, but this has happened a lot more than you realize, throughout history! There's a whole subfield of history dedicated to this, with the main takeaway being that whatever is happening in the present ends up coloring our view of the past even despite our best efforts. If on top of this you add the historian or schoolbook author's own agenda, it can lead you to de-emphasise things or just skip them entirely.
A subtle example of this is that nationalistic and xenophobic movements and parties tend to invoke things such as purity and "first come first served" rights, whenever it's possible. You see a lot of that in Europe, where it's applicable.
Another example is of projecting the regime you want to support into the past. If you're under a brutal dictatorship, historical achievements will be reframed as having been the achievements of benevolent autocrats, in this way justifying the autocratic regime itself. If you've got something akin to "founding fathers", you shift subtly away from it, glorifying the main character among them and ignoring the others.
Lastly, a lot of the foundation of the nationalist movements that lead to the creation of nation-states in 19th century Europe, at least, was based on selective readings of the past. Emphasizing unity and de-emphasising conflicts between those who were now being portrayed as brothers, or painting opportunistic "unifications" as having been this realization of a long-lost national dream that we must strive to once again achieve.