@Sharma, Vasundhra Thanks for your patience on this. I have checked with product team and below is the feedback shared from them.
Yes, the PII skill will only let you either return the PII found entities with no mask or mask the full length of the entity.
For partial masking some extra processing need to be performed. You can take the unmasked output of the PII skill and feed them into a custom skill and use a function as [explained here] (https://learn.microsoft.com/en-us/azure/search/cognitive-search-custom-skill-interface).
Feel free to reach back to me if you have any further questions on this.