After contacting Coda support they advised this is the intended behavior, for Coda to try to interpret the text column into something else, they refer to this as “auto-coercion”.
The data going into the column was a list of numbers, so we tried to use totext() to force the API to understand the string as a string, but this was not successful.
We were able to pass the correct string to the API but creating a mirror column that =totext(stringColumn) and then setting the API to pick up this column.
Not ideal, but it works!