VLookups suddenly malfunctioning on one computer only

Do the cells in question have blue triangles on them?

Without seeing the sheet, it's impossible to tell (even from the description), but my first guess is that the formulas are reacting to Numbers' recently-added support for arrays.

Some functions and references in a formula could, theoretically, return a range of values (called an array). However, previous versions of Numbers would only ever return the first item in the array and it may be that your spreadsheet depended on that (because you never had to worry about the extra values)

Newer versions of Numbers can return the entire array, which can be advantageous.

Sometimes it's ambiguous as to whether you want the single value or the array, so Numbers lets you define it, but that definition is missing in older files that pre-date array support.

When opening an older file, Numbers' usually makes the right call on how to interpret formulas that could return an array, but it's possible that the way you're using it gets misinterpreted. It puts a blue triangle on the cells that it thinks could be using a single value vs. array, as an indicator that you should check the cell for accuracy.

The fix should be fairly simply, but the specifics depend on knowing more about what the function is and what data it should be returning, which we can't see. Can you post more details including, possibly, a screenshot or example of the function you're using.

It's not so much that VLOOKUP() no longer works (or, at least, should still work), just that XLOOKUP does everything VLOOKUP() and HLOOKUP() does, and more,, including default values for missing results, so you can nix the whole 'IFERROR()' setup.

That said, it still isn't clear what's at fault here. From your spreadsheet, it looks like Q24 is doing the right thing... are you saying that should be some other value? Or is it some other cell that's at fault?

The only thing that might make a difference is that you're using a close match on the VLOOKUP, meaning that if the value isn't found, Numbers will return the next-largest value from the search column. Given the nature of the table I would expect this to require an exact match, and that would require a FALSE, or 0 in the last parameter. Indeed, the fact you're wrapping this in IFERROR() implies that you want an exact match, otherwise a close-match VLOOKUP would always return some value (even if not the value you want).

For reference, Number's VLOOKUP() definition states:

VLOOKUP(search-for, columns-range, return-column, close-match)

• • search-for: The value to find. search-for can contain any value, or a REGEX string.
  • columns-range: A collection of cells. columns-range must contain a reference to a single range of cells, which may contain any values.
  • return-column: A number value that specifies the relative column number of the cell from which to return the value. The leftmost column in the collection is column 1.
  • close-match: An optional modal value that determines whether an exact match is required.
    • close match (TRUE, 1, or omitted): If there’s no exact match, select the row with the largest left-column value that is less than or equal to the search value. If you use close match, you can’t use wildcards in search-for.
    • exact match (FALSE or 0): If there’s no exact match, returns an error. If you use exact match, you can use wildcards in search-for. You can use the wildcard ? (question mark) to represent one character, an * (asterisk) to represent multiple characters, and a ~ (tilde) to specify that the following character should be matched rather than used as a wildcard. REGEX can only be used in VLOOKUP if you use exact match.

I don't have specific suggestions because I'm having trouble following what you are showing in the table.

But that's a really "Excelsy" looking document!

Your life in Numbers will be much easier if you use multiple tables on a sheet rather than one big grid of cells like this. If you haven't done so already try having a look at the built-in templates at File > New in the menu. They are good examples of effective document design in Numbers.

And though the document may be new, you are using ancient functions like VLOOKUP. Why not use modern ones like XLOOKUP?

XLOOKUP - Apple Support

SG