Yes this is a problem... You can only generate past 12 months PPF statement from the date on which you want to generate it...
Any transactions before that, you cannot access through netbanking. You will have to contact the bank or update your passbook regularly.
Also, another drawback is that you cannot access PPF details through the HDFC Mobile Banking App. For all transactions related to PPF, you will have to use netbanking via any web browser.