Unfortunately It is a chronic problem of UE 5.0, It get less problematic until 5.3 but as grossimatte told, it got worse at 5.4.
Here we had to run in composition to workaround it, multiple hairs, all flickering depending on the scene and moment. Also the camera focus causes a great damage when the hair in first plane are blurry, so we need to render with camera focus disabled and then make the blur in the comp.
Recurring to composition breaks all the work pipeline, since UE were supposed to be the end-point, as Epic sell it. But unfortunately, the Epic Games Enginners didn’t even take a look here in the foruns, the only option to reach the people that will really help us with this engine-bug problems is paying the UDN fee.
Today I sat with my technical animatior to see what we could do about it, so we ran a lot of tests to understand what triggers that kind of flicker. We found that there is a limit of strands, about 3 million. Also we ran tests with different light sets respecting the 3mm limit, tomorrow we will finish the tests and will post here, maybe it could help you guys.