When you delete parts that you've created to listen to some of the other instruments separately, quite often the deleting of a part will change the tempo of the whole piece. Even though the tempo font and input will still read the same tempo as before.
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Please provide steps to reproduce and/or a sample score
Closed due to lack of information and inability to reproduce. Please report, with precise steps to reproduce, in a new thread if this still troubles you in the latest version (3.4.2).