In short, custom music discs will get cut off if they are longer than the Minecraft original soundtracks.

What I expected to happen was...:
I expected to hear the full length of my custom music (when playing a music disc), as I wanted to hear the full soundtrack. I made a resource pack that changed the music discs and their respective audio to hear music that wasn't from Minecraft. 

What actually happened was...:
When I played the custom discs on a juke box the music would get cut off at the times the original discs would normally end. Only discs that are longer than the original soundtracks would get cut off. Other discs that are shorter would keep playing the notes animation despite the soundtrack being over. 

Steps to Reproduce:
1. Create a resource pack that replaces the audio for records (music discs)
2. Play a disc in a juke box
3. Wait for the song to end or get cut off