Please provide the following information so that we can investigate further:
Are you using the default UI or custom UI?
If using the default UI, are you spotlighting users programmatically or through UI input?
If spotlighting programmatically, please provide a code snippet showing how you are spotlighting and include the return value of the spotlight method you are seeing.
It does seem to be intermittent whether the array is correct. It has occurred on multiple machines for separate users and different versions of macOS.
It also happens when multiple people are spotlit, then each user is unspotlit in turn, giving the correct array of users, until only one person is spotlit, at which point it will return a null array.
Again this is intermittent.
After several attempts, I have not been able to reproduce this behavior. How frequently are you able to reproduce this? Are you able to reproduce in the sample app as well?