Because currently watching more content from TVNZ+ which is 50.

I just changed mine to DV 60 will see if I notice any issues. Currently seems fine on TVNZ+ and YouTube. 

So the updated TVNZ app seems to have fixed this issue, but it's so broken in other areas as to be unable to fully test.

With the ATV set to 60Hz default, TVNZ will switch to 50Hz for 50Hz streamed content. Can't check how it does on 60Hz, because the only 60Hz content is on a live channel and the app crashes when watching any live content at all.