It seems to only affect recent models. Customers on the Samsung forums note that their UE50MU6100K, UE49MU7070 and MU6409 models either won't change the channel, or, worse, won't display anything after the splash screen upon powering on. We're waiting on an official statement from Samsung, but as of Tuesday (the problem began cropping up last Friday) customer service told forum members it didn't have a clue when a fix would go out to customers.
Earlier today, a moderator posted saying that the fix Samsung was working on was undergoing internal testing (it works, apparently) and that it would need to be installed by an "approved Samsung engineer." How long that will take to schedule and how widespread this glitch is isn't clear, but that could lead to more time without a working, brand new television and even more frustrated customers.
Update: Samsung has responded with the following comment:
"Samsung is aware of a small number of TVs in the UK (less than 200) affected by a firmware update to 2017 MU Series TVs on 17th August. Once this issue was identified, the update was switched off and we're now working with each customer to resolve the issue. Any customers affected are encouraged to get in touch with Samsung directly (1-800 SAMSUNG). We would like to apologize for the inconvenience caused to our customers."