I don't know anything about multiplayer, but could Minecraft be inadvertently checking to see if you have an active Nintendo Switch Online Membership when trying to connect to a LAN world?
I also have this issue, the standalone minecraft app is no longer in the Microsoft Store, it appears to be integrated into the new launcher.
I don't know anything about multiplayer, but could Minecraft be inadvertently checking to see if you have an active Nintendo Switch Online Membership when trying to connect to a LAN world?