TB 16 intermittent black screen on XPS 15 9570

Hi,

I have xps 9570(win 10 version 1903), latest bios (1.12), latest intel video driver(26.20.100.6952 ,A05), latest nvidia video driver( 25.21.14.2545 ,A05).

Asus VS248H-P connects TB16 dock through HDMI,  

Dell U2516D connects TB16 through DisplayPort.

From time to time, U2516D goes black for a few seconds and then comes back to normal. Asus monitor seems to be normal. This could happen several times a day, either idle or watching youtube. It is very annoying.

Any advice?