The role of a population job is NOT to detect new entities in a population - it is to detect entities behaving differently from their peers . If you want something like detecting new entities, then choose a temporal (non-population) job and choose by_field as the split (Relevant: ML Kibana: difference between by_field_name and partition_field_name and my comment on the notion of "dawn of time")
Also relevant: Temporal vs. Population Analysis in Elastic Machine Learning | Elastic Blog
Alternatively, if you want to detect something novel, you can consider using the rare function. See : Dec 4th, 2018: [EN][ML] Rarity Analysis with Machine Learning