Hidden rows after copy-pasting in Numbers on Apple MacBook Pro

That's going to be a very specific use case, and I'm guessing there's a logical explanation. I'm pretty sure that Numbers isn't in the habit of just randomly deleting arbitrary data.

When you say the rows are hidden, do you actually mean they're hidden? or are they suppressed?

If they are not displaying then I'd assume the destination table has filters set and that the pasted data doesn't pass the filter test - e.g. Show all rows where 'Name' is "Bob" or "Sue", hence anything to do with "Paul" is being filtered out - in other words, Numbers is doing exactly what it's been asked (told?) to do.

You need to look more closely at the setup of the destination table, specifically Inspector -> Organize -> Filter.

If I'm wrong and there are no filters set, then please feel free to post back with more details (including screenshots which often help triage these kinds of problems).