Sadly this is a normal thing to happen. It's a limit you hit, and then the tree gets cut off and starts again.
There are some programs/sites where you can keep track of your Sims family trees, I suggest Googling.
But it's only on the seventh generation? Also, the tree didn't start over (thankfully!). The founder is still there and almost anyone else. I just have to press the missing sims' parents, and then the rest of the tree is stille there. Only the information on two sims disappear which makes the tree look weird.
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