Songs Disappearing from iPad

I have a 128 GB iPad Mini (WIFI only) with over 40 GB of free space. I have a large physical music library, and I have to manually move them from my computer to my iPad in order to have them. I have noticed REPEATEDLY that some songs or entire albums disappear entirely from my iPad's music library. They still show up under "On My Device" when looking at the device in iTunes, but once you click on the tracks, they show an error saying that they are not found. This is obviously a huge issue, but I haven't seen that anyone else is experiencing such a thing. I don't understand why it is happening, and I would appreciate guidance regarding this situation. Also, yes, I click eject and wait for it to disappear from iTunes before unplugging the device.