Mac: Copy/pasting from app to text editor pastes all lines, but Excel only the first line

[ohrenkino]

See the documentation on how to export text to the clipboard:

Export Tags to Text Files

Export to Text Files Mp3tag lets you export tag and file information to text-based formats via File → Export or ⌘E. You can use any text along with the available placeholders and scripting functions to define the export format. Mp3tag applies this format to each selected file, generating a textual representation for each one. The resulting export file contains one line of output per file. You can define a format string used to create the export filename at Preferences → Export. Export to Clip…

Check which format string you have there and whether the output format is supported by the Excel import functions.
Perhaps the tab character ($char(9)) is a better separator.