+2 As I saw at least one other report besides mine:
Mine: [Regression] (550.54.14) Native HDMI 2.1 signals i.e above 4k60Hz won't work on 8 bit output (hdmi_deepcolor=0)
Other one: [SOLVED] 550.54 & 550.67 break 120hz on LG C2 TV Monitors; 545.29 is last working version - #10 by saschaheid