The KF drive was first successfully used in the 2100's. Leaving time for development, and colonisation, and the earliest you could believably have a colony ship run off into the stars and dissapear (for your proposed planet, assuming that its beyond the borders of the inner sphere) is, lets say 2200.
If your fic takes place in the modern era- lets say 3000- that gives you 800 years for a colony to fail, and raise itself up again.
So, the colonists arive in the periphery. They land. (Since their ship left around 2200, this predates dropships, and they would have had to use shuttles to move stuff from their jumpship to the planet). They start to build a colony.
Then something happens. (Exactly what is up to you- natural disaster, plague, war, they all decided to become Amish) As a result, their advanced technology is destroyed or lost.
Time passes. They spread out. They make farms. With the lack of advanced medical technology, lifespans shorten. At the same time, they have more and more children. (While in modern days most western families are small, children in developing nations -like your colony- have as many children as posible. Partly because they can't afford contraceptives, but mostly because children are usefull assets, able to help farm and provide for the family.) This causes most history to be lost, as each generation remembers less and less of what their parents told them.
Then, one day, something changes. They have an industrial revolution.
In Real Life, the industrial revolution took about 200 years, and the world's average per capita income increased over 10-fold, while the world's population increased over 6-fold. If your proposed planet did the same thing, it would explain a lot.
So, 600 years for a colony to fail, turn into a bunch of 10th century farmers, and loose all their history. Then 200 years for them to have an industrial revolution, rebuilding back up to a WW2-era technological base.
Reasonable. Especially if your planet has internal conflict- nothing encourages technological development like a good enemy.