Can't connect to another Switch multiplayer world.

When I'm logged into the Microsoft Account/XboxLive on my Nintendo Switch I can join other Windows10/mobile crossplay worlds, but I can not join other Nintendo Switch worlds. 

People can join my hosted Switch Worlds with their Windows 10/mobile device, but not their Switch. Switch to Switch does not work. Once the game tries to connect to a multiplayer world, it fails and says can't connect to World.

I already changed all my internet settings, NAT type, opened ports (XboxLive and Nintendo ports), and tried without Microsoft Account (Nintendo Profile to Nintendo Profile) etc., still won't work.
NOTE: The previous "Minecraft: Switch Edition" works without any problems, I can connect to other Minecraft: Switch Edition multiplayer worlds instantly, and I can also play any other games online. It is literally only the bedrock Minecraft version that has issues.
NOTE 2: Yes, all Accounts (Nintendo, Microsoft, XboxLive) have perfect settings, that's why I can play everything else online.

The game updated to Version 1.6.0, it is still not working.

Additional information added by [MCPE Mod] Auldrick:
This issue appears to affect Switch-to-Switch connections via the Nintendo Switch Online network as well as via Xbox Live.