Whenever you install multiple addons with player.json, only the top player.json addon works and it seems to override all of the other addons with player.json.. This can be fixed by changing the addons & player.json API. I hope this gets fixed soon, because it really is a hinderance to massive modded worlds. I've noticed that this has been a problem since 1.12.0 and hasn't been fixed since.

Steps:

1. Install multiple addons with player.json
2. Enable experimental mode
3. Test out the addons under the top player.json addon to see if they work