The specific conditions under which the Starts voice property takes effect are complicated. Without auditing the code or asking the developer who originally implemented it to give me his best recollection of all of the conditions, I can’t tell you for sure.
The fundamental complexity is that, in common with some other properties, the Starts voice property does not operate atomically, i.e. it depends on the properties set on other notes or rests in the same voice.
This is the main reason why we provide the Edit > Remove Rests command, as it can work out the specific combination of properties that need to be set, potentially at great rhythmic distances, in order to remove the rests you don’t want to see. I strongly recommend using that feature instead of becoming frustrated with the Starts voice property.