Using 19.7.1, but this starting happening a few builds earlier.
Basically if I swap user accouts, the screen goes really dim.
Toggling the monitor off/on restores the brightness.
Also, when logging back in from resume, often the screen while being dim now has a nice pink/white overlay.
Rebooting sovles this.
Vega64 LC + LG34GK950F monitor.
HDR and FreeSync2 enabled.