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.
Airtel is so big they use roaming for their customers. If you have a SIM from Bengaluru and go to Delhi you'll see the little R indicator. That suggests the Airtel business in each state manages at least parts of their network independently. And so the MitMs could be deployed non-uniformly.
India used to be and still is split into several telecom regions with different spectrum leasing, operations and governance.
Until around 2009-10, when you are traveling out of state, you had to pay roaming charges. Worse used to be metro cities within their own states as they used to be different telecom circles. I used to pay roaming charges when going to Chennai from rest of Tamil Nadu. Even the operators were different sometimes. E.g. there was no Hutch (now Vodafone) originally in rest of Tamil Nadu and they operated only in Chennai. Similarly RPG (later Aircel which went bankrupt couple of years back) had 2 networks - RPG in Chennai and Aircel in rest of Tamil Nadu. It used to be a mess.
No operator had pan-India operation as every small operator had their own fiefdoms and the big operators like Airtel used to pay roaming charges to those operators for their subscribers to get signal.
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