Each line within a FileMaker multi-line field is delimited with a vertical tab character, which is not supported by Ninox. I just created and used the following Ninox global function to replace all the vertical tab characters in each field for every Ninox record. I added this global function under the Ninox "Options" tab, which is located to the right of the "Data model" tab. The function is called by adding replaceVT() to a button. Just replace "TableName" with your table name and modify/add lines for each of your Ninox multi-line text fields. Seems to work well. Hope this helps somebody else. Be sure to back up your data before trying it!

function replaceVT() do
  let v := "
  ";
  for i in select TableName do
    i.('FieldName1' := replace(i.'FieldName1', "", v));
    i.('FieldName2' := replace(i.'FieldName2', "", v));
  end
end