When an MCC has a lot of volume moved, banks internally validate the MCC. They must have notified SBI Unipay about it and SBI has a very slow change process implementation, so they just hid the option in the interim. An MCC correction is most prob in the pipeline. If the bypass is heavily used, the MCC correction might be pushed sooner. I would urge the Unipay Users to limit their bill payment to some extent but this is India and we have an inner programming to not listen until a good thing is burned down to the ground.
And moving forward all bill payment portals will be mostly free of any rewards because banks know how it's being abused. We might get something temporarily but it's done now.