We often get the message: "Whatever I set in the system template, I always hear 'Piano' and not what I have chosen." What happened?
Hidden in the depths of the sound configuration (in the capella-tune dialog) you can select a "standard sound"; if you don't change anything, that is "Piano".
At the bottom of the dialog you will find the setting "Same sound for all instruments". Perhaps you have set a check mark here? You only notice the consequences when you don't want to listen to the piano anymore. This check mark is the cause of the problem.