Hello,
thanks to technical support of my distributor, the problem is solved.
The reason was very simple:
The HL6528RD and the HL8548 have a disabled sleep mode of the USART by default:
But the HL7692 has an enabled sleep mode 
!!!
The firmware used was the BHL769x.2.16.170400.201702221601.x7120m_1
I think, it would be very time-saving, to have a list of deviations
inside the HL-family - or better: no deviations.
Regards,
Moardy