Discussed with KK so I thought I'd capture this idea as a ticket.
Currently buckets are made using excludesFile, excluding all tests from other buckets.
If you have a new test, it won't be part of any bucket, so it won't be mentioned in any generated excludesFile and so it will be executed in all buckets.
The idea would be to generate inclusions files instead, for all buckets except one that would still use the excludesFile. That way, new tests would be assigned to only one bucket.