Problem with search function in Numbers on Mac

Most of your screenshots have Table 2::A2 empty. With that (the null string) as the search string, apparently the result is 1. I would have expected an error. There is no correct answer to that search.

In the screenshots with blue triangles, look at the bottom of the screen. It says "Werkelijk" which I assume translates to "actual" in English. There is no formula in those cells, just a number 1. I believe the blue triangles are probably warning you that the formula was replaced with the last value. It appears you added those rows when the table was filtered (the filter was on). Formulas that affect the filter will not flow down into new rows when the filter is on. You need to turn the filter off before adding new rows to the table.

I can't reproduce the problem from the available data.

From what I can see, the search formula in C2 is valid. Is that formula replicated/filled down? It seems to me there must be a different formula in C3::C11