API getRecords method with lastModifiedTime doesn't work in some cases

API getRecords method with lastModifiedTime doesn't work in some cases

Hi , 

We are using getRecords method in order to fetch data from our CRM account. In particular we are using getRecords with lastModifiedTime paramenter in order to fetch records in delta mode as you explain in your documentation: 
https://www.zoho.com/crm/help/api/getrecords.html#e2  

Now the problem is that in some case the delta extraction doesn't work and some records that we have certainly changed are not fetched. 

We suspect that this happens when we change the values of individual fields without changing the entire record in edit mode (please see attachment). 

Could you kindly check if it is a bug?

Thanks in advance.