I cannot sign in to multiple profiles using the Legacy Game Controller Adapter!
The Xbox One console recognizes the Legacy Game Controller Adapter as a single controller for a single user. Therefore, the adapter is able to support one profile sign-in for all legacy instruments. That one profile will be assigned to all connected instruments via the Legacy Game Controller Adapter. Rock Band 4 controllers can continue to sign in to their desired profile as intended. Unfortunately, there is no way to change how the Xbox One console reads the Legacy Game Controller Adapter and allow legacy controllers to sign in to multiple profiles.
If you are experiencing issues with the game while attempting to sign-in or switch between multiple profiles in Rock Band 4, please quit the game, sign-in to your desired profile, and restart Rock Band 4 only when you have signed in to the profile you wish to play with. Signing in to your desired profile before starting Rock Band 4 will help avoid any potential issues that may arise while in-game.