This is not about the block a month ago. This is a new block.
When it was blocked a month ago, there was no notice. Now, there is a notice that it is a TRAI order. usually seen on sites that the govt themselves ask to block (piratebay, torrentz.eu, etc)
I guess its only limited to some regions. I'm using Airtel for my home connection as well mobile and get redirected to https everytime I visit the http version of ddg on both connections.
P.S. From the screenshot, it looks like you're trying to connect to [http]://duck.., hence shifting to https works.
This also hints towards a mixture of plain old http censorship and https censorship, which Airtel (in fact all ISPs) do randomly