Shimano Hub automatically switches off while riding

I have few Shimano dynamo Hubs and I had to face this issue on several: they seem to be on good working condition. I checked them on a truing stand with a light and they worked fine. Then when I start riding them, they light at low speed, then turned off automatically reaching few miles per hour. Finally, after few miles, they didn’t even light at low speed.

Do you have any clue to help me fixing this issue ?