I don't know if this issue lies in gnome-settings-daemon, mutter,
gnome-shell, the kernel, or somewhere else.

Some time in the last month or two, something changed such that I can no
longer use 4k with mirror mode over HDMI to an LG OLED TV (tested with
both C2 and G2). Instead, I'm limited to 2560x1440. If I use "join"
mode, I can set the TV to 4k, but mirror mode no longer allows 4k.

My laptop screen is 3840x2400. In the past, mirror mode worked at
3840x2160 and just letterboxed on the laptop screen.

ThinkPad X1 Carbon 9th gen. Intel graphics. Happy to provide more
information.