Export multiple by labels may add unwanted silence to exported files if multiple tracks are present that don’t all start at time zero. You can workaround this by using Edit > Select > All then Tracks > Mix and Render to render the audio to one track, or by generating a short amount of silence at the start of each track that is offset from zero.
This will be fixed in the next 2.1.3 version of Audacity when released later this year.