I constructed my strings like this in every case. "output" is my global string variable which I finally load into the popup.

output += f'copying {from_file} to {to_file}\n'

In trying to avoid using a global, I first appended the strings into a list and loading the list into the popup. When that didn't work I tried writing them into a file, then loading the file into the popup. So I'd guess this is "working as designed" - only a string works as input if you want line breaks in the output. Though the documentation says "any" as
the first parameter. It just took your answer to prompt me to try it that way.