Nether portal sound behaves strangely when using custom sounds

Typically while standing near a Nether portal, when the "portal" sound plays, the sound fades when the player moves away from the portal. However, when using a custom sound in a resourcepack, it does not fade like it is supposed to. It is possible this bug may affect other sounds as well, but I have only noticed it here.

I also tested this on version 1.12.2, and both sounds functioned normally.