It should work as described in the tutorials and the reference manual. If it's not working that way for you, you may want to submit a support ticket with the following information...
- Type of track and media involved (audio, midi)
- Operating System and Studio One version
- Make sure to check that the media on the inactive layer being soloed is not muted.
- Also express if this happens only in one particular song or happens more universally across multiple songs.
- If closing and re-opening the song stops the behavior. (perhaps a temporary glitch or misbehavior)
If possible, please also attach an image of all of the expanded layers that don't play in solo so the tech support representative can see them.