How to determine what entity changes Xorg DPI settings?

I've installed Xubuntu 19.04, and to account for my laptop's screen resolution, in /etc/lightdm/lightdm.conf I've added the line

xserver-command=X -dpi 166

The result is that XFCE panel, terminal and other GTK2 and GTK3 programs take on the correct font sizes (the same as if I had set _Custom DPI setting_ in _Appearance_→_Fonts_ to 166), but all the others like e.g. Yakuake still see 96 DPI. Moreover, if I try xdpyinfo, I do get this 96 DPI:

$ xdpyinfo | grep dot
  resolution:    96x96 dots per inch

Also, if I tick and untick back the _Custom DPI setting_ option, the fonts shrink back in XFCE too (apparently, DPI gets re-queried from X server). So I suspect that, during session startup, some entity changes the screen DPI setting as if by running xrandr --dpi 96. If I manually run xrandr --dpi 166, programs start behaving correctly. I'd like to find out what exactly entity is doing this, so as to fix this at the core instead of adding post-startup workarounds. How can I find it?