Catalina bug: external Thunderbolt3/USB-C monitor briefly blacks out after signal connection with MacBook Pro

[dogjw]

After Catalina migration, on a 2018 MBPro 13" i7 in closed clamshell mode, the external display (connected via Thunderbolt3/USB-C) now briefly blacks out about 10 seconds after the MBPro establishes a signal connection to the external monitor, such as:

1. after booting the macbook pro with the external monitor connected, or
2. after "awakening" after the external display has auto turned off (from Energy Saver settings), or
3. after connecting or powering-on the external monitor connected to the MBPro via T3/USB-C cable.

The brief (1.5 second) blackout usually occurs only once upon the signal connection and doesn't repeat until the external display auto-turns off again and then is re-awakened. The problem also does not occur on the MBP built-in display - only the external monitor. Resetting SMC/PRAM doesn't help.

This worked perfectly in Mojave and seems to be a 2018 MBP external display driver bug introduced by Catalina.

Equipment used:
2018 MBPro 13" i7 (in closed clamshell mode) (Intel Iris Plus Graphics 655 1536 MB)
2018 Samsung C34J791x external monitor (connected via Thunderbolt3/USB-C cable)

 

[dogjw]

This is still not fixed after 10.15.1 update.

Is anyone else experiencing this issue?

 

[Weaselboy]

I'm seeing this on reboots, but not on wake. I use the LG 5K monitor and once it is booted and I see the desktop it will black out for a second or so then come back and all is well afterwards. I never noticed this happening under Mojave.