Using Windows 10     
Software Version 25.5.0.290

I have a score entered from scratch - i.e. not imported.  I set an initial tempo of crotchet = 55 and that does not change throughout the piece until a 'rit' and then a final 'pause'. All well and good. It plays back perfectly.

But I extract the 'cello part and play it back.  Suddenly, half way through measure 6, it takes off at a new tempo. I didn't change anything. It maintains the tempo when I play back the full score.

1. What's going on and how can I prevent it?

2. As I do need the sound recording of the 'cello part, is there any way of muting the other instruments when playing back from the full score, as it works fine there.