There are a few possibilities. Check your skin.ini file. You might be missing the tail image which NoteImage#T: is pointing to, or NoteImage#T: is pointing to a transparent asset.

Mania skin.ini reference (ctrl + f and search for NoteImage#T:)

If the line is not there, then the game is looking for the default file name, which is mania-note#T, so your problem lies there.

Mania long note wiki reference