I FINALLY found a solution of this problem. Set the HDMI 1.4 mode on your external monitor. If you have this problem like a shit, NEVER use HDMI 2.0 mode. I've got peace FINALLY.
(I'am using macbook pro 15-inch 2017, i7 2.9Ghz, 16GB, and Radeon Pro 560 and Catalina 10.15.0)
Last edit date: 2019.10.24