[Alf Stewart]

It's obvious - it was the most obvious in the current Laura K docu-series "A State of Chaos". They used clips of Dimbleby announcing results for the 2016 Brexit referendum and the exit poll of the 2017 General Election, but for the 2019 General Election they used a radio clip for the exit poll and no studio clips of Huw announcing any results, just clips of Laura doing analysis.
Huw has been front and centre of news coverage of the political chaos since 2016 but not one clip of him has appeared on the program. 

Plus there was the incident this weekend on Breakfast where an old episode of Newswatch was played out by mistake and as soon as Huw appeared they hastily took the program off, blaming "technical gremlins".

I don't think Huw appearing on Newswatch was the reason it was pulled off air, more the fact it was seven years old and the wrong episode. Huw appearing may have been the trigger that made someone realise it was the wrong episode but beyond that I have a couple of straws for you to clutch.