EOS R Horizontal lines across picture at 1/2500 or faster

[Cherkesov]

Hi guys, I'm looking for a competent answer. I bought a Godox QT400III for photographing water droplets and found that starting from 1/2500 and up to 1/8000 of a second, I get horizontal lines in the picture across the entire field of the frame.

At first, I decided that the problem was in the Godox flash. Then I checked my camera with Yongnuo HSS speedlight and YN622C HSS synchronizers. There is also a problem with horizontal lines with this flash.

Then I checked the Godox QT400III with my old Canon 6D SLR camera and the Canon M50 mirrorless, everything is fine up to 1/4000 of a second (these cameras can't shoot shorter than 1/4000).

Who knows what the problem is and is there a solution?

P.S. Firmware versions updated

[Anonymous]

It could be a failing shutter on the camera. How is it with no flash at those shutter speeds?

[Cherkesov]

Without a flash, everything is fine at this shutter speed. With constant light, everything is fine too. The problem is only with the studio strobe flash. 

[Anonymous]

Did your flashes used to work ok at those shutter settings? I assume that you have the flash set for high speed sync. Do you have a Canon flash to test?

[Cherkesov]

Flash in HSS mode. Unfortunately, I don't have a Canon flash to test.