Never. You might want to organize your music, and make sure it's all in the same place. Consolidate your music.
This should be something that you should be able to ask Siri.
"Why does iTunes delete my music"? REF: https://forums.macrumors.com/threads/1316830/
EDIT:
Looks like Apple is anti-motoleo. They are deleting your iPhone apps too. https://forums.macrumors.com/threads/1317622/
Your iPod Touch is broken: https://forums.macrumors.com/threads/1317094/
Hope you get your issues fixed.
Bahahaha! Nice!