Jon, I’m happy to tell you that this isn’t a bug, but a ‘feature’ that you can turn off in the in-game settings.
It’s called ‘Workout Pain Effect’ and distorts your screen in the same way that your eyesight is supposed to be affected (negatively) when doing really hard efforts.
When riding, go into the ‘Menu’, go to ‘Settings’ and un-toggle ‘Workout Pain Effect’.
Sorted.