It would be enough if computer would check steering wheel angle to determine if you are in the corner or going straight. 

I don't think it has anything to do with steering behaviour. The CCS is using wheel speed information from the ABS computer to decide how to act. In a case like this it's seeing a significant error between wheel speeds causing it to react. Aside from OP's case where it was a different tyre brand, this could occur in an aquaplaning scenario as well. I'm a little surprised that it didn't just drop out of cruise control though that could cause problems where the driver is not paying attention. This gear-changing behaviour might be its way of communicating to the driver that something is off. I think I'd prefer it to just put up a warning on the MFD.

Just in case anyone experiences the same and looks at this thread.....

this condition is caused by having a flat(ish) or way under inflated tyre on the car.

my mk6 Golf gtd does exactly this.

it isn’t fitted with one of the new flat tyre monitors but if it were it would trigger that too.

i drive high motorway mileage each week (1400 miles) and if I get lazy and don’t check tyre pressures each week this kick down on cruise (particularly when on flat or going down hill) is how I know to get pump out.

😀