Sadly the exact same issue here.
iMac 2009, Sierra, NOW TV 2.1.4 - no extra screen connected.
Player looks fine in small window but when I scale to full screen in distorts.
I do regret having signed up for 4 months now... don't want to watch in a small window!!
A quick fix would be highly appreciated!
been in touch with the support but they couldn't help either.
Instead they told me to wait for an update or software fix on their end...
as to when this would be they could tell... seriously a bummer! and certainly no customer orientated support.
I'll watch this for another day or so and cancel my subscription.
Are you able to expand the window to fit the screen instead of going full screen? Does that stretch the picture, or is it just fullscreen mode that's the issue?