Microsoft NLB doesn't provide a full proof fault tolerance. A node will be considered up as soon as the TCP-IP stack of the machine is up and running whereas the ADFS service might not be running yet.
That said, if you turn off complete one node, you should expect the authentication to work on the second node... I would start by creating an entry in your HOSTS file to make your ADFS FQDN point to the secondary node in a normal operation mode (2 nodes are up). And see if authentication works. If it does work, you have a NLB issue. If it doesn't work, you have an ADFS issue and we'll need to check if the database sync works properly and what not. For that second part, you can use these tool: Diagnostics Analyzer.