When using the dialing device, you can see and dial to private matter receivers that you are not a member of. Looking at the 1.16 branch, it seems that the issue is here. If the player is not found in the dimension, it defaults to true.
I believe that changing the line to PlayerEntity playerByUuid = level.getServer().getPlayerList().getPlayerByUUID(player); would fix the issue.
When using the dialing device, you can see and dial to private matter receivers that you are not a member of. Looking at the 1.16 branch, it seems that the issue is here. If the player is not found in the dimension, it defaults to true. I believe that changing the line to
PlayerEntity playerByUuid = level.getServer().getPlayerList().getPlayerByUUID(player);
would fix the issue.