Your test files package is helpful to open the view to the situation.
There has been similar problems ...
/t/4623/1
/t/10690/1
... and explanations ...
https://support.microsoft.com/en-us/help/17...eeding-max-path
https://support.microsoft.com/de-de/help/17...eeding-max-path
Hmm, I gave your example a try and can confirm, ...
when loading the m3u8 playlist file, then Mp3tag cannot load the file ...
... but the other file will be loaded ...
test.mp3A new test file within the m3u8 playlist file, having the relative path ...
..\..\Komponisten\Saint-Saëns, Camille (1835–1921)\Symphony No.3 C minor, Op.78 »avec orgue«\Boston Symphony Orchestra · Charles Munch, Conductor · Barj Zamkochian, Organ (RCA Red Seal 1959)\01 · I. Ad.mp3... will be loaded too.
For such problematic case of not respecting such long path names ...
I have a Mp3tag tool defined, named as "Load this folder in Mp3tag", ...
Path: <... the path to mp3tag.exe ...>
Parameter: '/fp:"'%_folderpath%'"'
Pointing the cursor to the succesful loaded file "test.mp3" and applying the tool will load all files from the folder.
I do not know whether there is a way to implement a similar method into Mp3tag to respect such cases when loading a playlist file.
DD.20171010.1246.CEST