This sounds like I've an electrical issue, but no idea of next steps.
I have a Gazelle Tour Populair with what I believe is a Busch & Müller LYT Lumotec Retro front dynamo-powered light.
When starting to move the light comes on, but quickly cuts off for the rest of the journey.
The shop have already replaced the light with a new one, and same issue.
Could it be a power build-up somehow?