Error : Unable to find column 'separateID', did you mean 'separateID'? | It's an unusual experience.

[leinharte]

It's an unusual experience.

An error occurs when the following formula is entered in the column's App Formula.

IF(
        [separateID].[foreignCurrency?],
        [amount] *
               IFS(
                      [ceclassification ID] = "bigid51B0A476",
                      [term end exchange rate] - [average exchange rate],
                      NOT(IN([ceclass ID], LIST("bigid51B0A476"))),
                      [period-end exchange rate] - [spot exchange rate]
                       ),
               0)

The error message is:
Unable to find column 'separateID', did you mean 'separateID'?

BTW, if I change the part of  the formula "[ separateID].[foreignCurrency?]" by clicking "Data Explorer", in fact the formula content is the same, the error disappears.

Why does this happen?

P.s. FYI, if you copy and paste this formula into Google Docs and then copy and paste it into App Formula again, an error occurs.

Thank you