Additional confirmation. YES. For some reason, it's true that having someone else already logged into the realm somehow allows the switch to see the realm enough to connect to it at that time only. Just tested it. Not a fix, but hopefully narrows down what the problem is by narrowing down what works. 100% certain it's 19.1 that's the issue though while 19.0 had none at all.
Confirmed. It would appear that 19.1 is not compatible with realms, but 19.0 still is and works just fine, hence the switch no longer connecting even though everything else connects just fine.
Additional confirmation. YES. For some reason, it's true that having someone else already logged into the realm somehow allows the switch to see the realm enough to connect to it at that time only. Just tested it. Not a fix, but hopefully narrows down what the problem is by narrowing down what works. 100% certain it's 19.1 that's the issue though while 19.0 had none at all.