I am having the same issue. I found this on another forum related to Brevo and a user having a similar issue. Not sure if it helps anyone find a solution. I am not a developer, just researching.
"It denotes that the data is an array i.e Multiple Lists can be there for given data. So, In sendblue since it is a multi-select i.e an array, you need to pass an array of number not a single number.
To do this, what you want to do is,
Pass the List ID as an array while in map mode
Or, If you want to pass a single data then you need to use formula to create an array, add(emptyarray;data)