Basically he kept disappearing and reappearing.
Mm, I never use a flashing light on its own for this reason (except sometimes in London in the daytime, when I’m perfectly visible without a light but I can expect to be filtering past queuing traffic a lot, and I know from experience that a flashing light is more noticeable in a wing mirror).
A year or two ago, in the pitch dark, I was about to pull out onto a roundabout (in the car). Empty roundabout, nothing approaching.Fortunately I’m the sort of person who always looks twice, because when I looked first time nothing was visible, because the chap on the bike who was just approaching my arm of the roundabout had a flashing light which was off when I looked the first time.
That said, I’d not entirely agree with everyone saying “you need a flashing and a static light”. I’d suggest you “need” a static one when it’s dark. You might feel that a flashing one helps in some situations, in which case great, fit one. I think it’s excessive to say you “need” one.