XPS 15-9560, dual U2518D, color bit issue

When daisy chaining two U2518D with an XPS 15-9560, the second U2518D displays 6 bit color depth?

Configuration

XPS 15-9560 USB Type-C out port -->

USB Type-C to DP converter -->

U2518D DP in port
U2518D DP (MST) out  port -->

DP to DP cable -->

U2518D DP in port

Both display 8 bit when used independently.

Is there any known GPU driver issue?

Thanks.