HR of your current company could have shown you as "Casual Employee" from 28th Aug 2018 to 31st Aug 2018. To avoid overlap, your appointment could have been shown on 1st Sep 2018. It appears that your current HR has misguided you by not doing so.
Anyway, now you have two options. One is to talk to your current HR and shift the DOJ from 28th Aug 2018 to 1st Sep 2018. If he says that he cannot do that then you need to go back to the previous employer and ask them to show your Last Working Day (LWD) as 27th Aug 2018.
26th May 2019 From India, Bangalore