The bug

When "replace": false is used in a sound definitions file (or the replace option is unspecified), the provided list of sounds is supposed to be added to the original definition from the lower pack (including vanilla). This apparently no longer works, and the entire list of sounds is always overwritten.

How to reproduce

1. Download the attached world
2. /playsound beacon.activate
   →  It always sounds like a lantern breaking instead of 50% lantern, 50% the original sound

sound_definitions file

{
   "format_version": "1.14.0",
   "sound_definitions": {
      "beacon.activate": {
         "category": "block",
         "replace": false,
         "sounds": ["sounds/block/lantern/break1"]
      }
   }
}