This is a known behaviour, or rather a design set forth by Azure where, when a field is set to null/empty, Azure will not trigger a request to the downstream system.
Here's a forum post regarding the same: https://learn.microsoft.com/en-us/answers/questions/1160586/how-to-synchronize-that-a-field-has-been-emptied?cid=kerryherger
Since Azure does not send the request to nullify/empty the field, the existing value remains the same on Freshservice. However, changing the field to a different non-empty value would update the same on Freshservice.