I think there may be something going on behind the scenes here that will be a bit unintuitive – before I waste time explaining it, could I have you verify my hunch by changing your formula to this:
Great! So I’m guessing you made the deduction already that behind the scenes, you were passing an array, rather than a string, to the FIND() function (which is expecting a string). If the array happens to have only a single element, then apparently Airtable’s formula engine is happy to coerce that array into a string for you. But if the array has more than one element in it, it seems the formula engine wants to leave it as an array.
My solution above just brute forces the array into a string by concatenation. But apparently, if your solution does indeed work, you can also explicitly declare the array as a comma delimited array, and that will allow it to be coerced into a string by the formula engine (which seems odd, but if it works, it works!).
