Running current TW, using gnome preferences to set idle-dim does not affect the
value in gsettings, even after restart.
Using gsettings to set org.gnome.settings-daemon.plugins.power "idle-dim"
directly from true to false, Gnome still dims even after restart.
The only fix was setting idle-brightness to 100 so dimming isn't noticeable.

Possibly upstream bug, only reference I found was a closed Fedora 29 bug that
couldn't be reproduced.

Using Thinkpad T480 model 20L5 with dGPU, xf86-video-intel with kernel option
"nouveau.modeset=0" (nouveau crashes with MX-150 GPU)

No relevant errors in dmesg or journalctl -q.

Bug shows up on Gnome on Wayland and Xorg.