Rome was highly organized- but not ideal.
Even the Roman Empire did not escape without losses. And when it did, they did not go back.
First, the Legio IX Hispana. A strong, famous army unit. Then suddenly--gone. And at about 120 AD thousands of soldiers simply disappear into history. No clear battle. No explanation. Either they were disposed of... or transported away somewhere.
And then there are the Sibylian books. They were semi-secret books to be used during large emergencies, such as war or disaster. Leaders trusted them. Then one day, they were burned. Now we can not tell what they had in them.
And then it gets strange.
In Liqian, there are individuals who appear dissimilar to those around them, in terms of being light-skinned, which is not typical of the place. DNA portrays some western ancestry. One of these is that Roman soldiers who had lost their way, had landed there many years ago.
Here's the simple truth:
Even a mighty empire is able to forget what happened.
Armies disappear. Knowledge is destroyed. People move and mix.
And with time, history is something unfinished.