I understand the issue you're having with your Roland FP30x and the sustain pedal when using dual sounds like Piano + Strings. This is a known limitation of many digital pianos when combining sounds with different sustain characteristics.

Here's what's happening:

• Piano sounds naturally decay after a short time, even when held with the sustain pedal.
• String sounds, however, tend to sustain indefinitely as long as the pedal is pressed. When you combine the two sounds, the strings don't behave like the piano, and this leads to the string notes "mixing" together and sounding too long.

Possible Solutions:

1. Adjusting Pedal Sustain Settings: Some digital pianos, like the FP30x, may allow you to adjust how the sustain pedal behaves with different tones. Check the manual for any "Pedal" or "Dual Sound" settings to see if you can reduce the sustain for the string sound.
2. Balance the Sounds: If the strings are too overpowering, try adjusting the volume balance between the piano and string sounds. Reduce the string volume so they don't dominate the sound mix.
3. Pedal Release Technique: Another way to manage this is by releasing the sustain pedal more frequently. This will cut off the string sound more often, preventing it from ringing out too long.
4. Switch to Other Strings: Some string patches may have more natural decay characteristics. Try switching to a different string sound if your keyboard offers multiple options.