xorg ignoring custom configuration

Setup:
* Lenovo Thinkpad T570
* Xubuntu 18.10 (XFCE + LightDM)
$ lsb_release -rd
Description: Ubuntu 18.04.1 LTS
Release: 18.04

* X.Org X Server 1.19.6
* Kernel: 4.15.0-43-generic
* 1 internal laptop display, 2 external displays plugged to a Thinkpad Docking Station (which acts as a simple port replicator)

Problem:
This is part of a bigger challenge in which I try to get Xorg to load two specific config files in a running Xsession, each containing one xrandr command.
Managing Display Configuration with xrandr scripts works reliable until xorg tries to set the configuration itself.

I may be overlooking something here and I'd be glad if you point me to the right direction, but I'm completely lost at this point. Although, this is far from expected behaviour, so I'm reporting it as a bug.

Situation:
When docked with two external displays, the internal laptop display gets forcefully cloned every time I dock the laptop.

Even when setting and saving the configuration either with arandr, xrandr, ~/.xprofile or XFCE autostart-scripts, Xorg seems to either ignore or overwrite it. Said scripts work fine when manually executed.

Example xrandr configuration:
xrandr \
--output eDP-1 --mode 1920x1080 --pos 3840x0 --rotate normal \
--output DP-2-1 --mode 1920x1200 --pos 1920x0 --rotate normal \
--output DP-2-2 --primary --mode 1920x1200 --pos 0x0 --rotate normal \
--output HDMI-2 --off \
--output HDMI-1 --off \
--output DP-1 --off \
--output DP-2 --off \
--output DP-2 --off

Steps to Reproduce:
- Dock Laptop to Docking Station (Xorg sets internal display to clone to DP-2-2, while DP-2-1 is set correctly)
- Fix and save desired Display Configuration (either via arandr, xrandr, ~/.xprofile, setting autostart-scripts ...)
- Undock Laptop
- Dock Laptop again
- false xorg-defaults are loaded again: cloning the internal display instead of extending it or loading any previous saved settings.

No reboot or logout required, it just breaks.

I've tried several solutions but none of them work, so I'm open for suggestions.
- deleting monitors.xml has no effect
- xorg.conf is non existing
- no Errors found in: /var/log/Xorg.0.log, although I find:
[ 16392.649] (==) Using system config directory "/usr/share/X11/xorg.conf.d"
[ 16392.650] (==) No Layout section. Using the first Screen section.
[ 16392.650] (==) No screen section available. Using defaults.

Alas, there's no defaults configuration either, let alone one that states that only the internal display shall be cloned:
# ls /usr/share/X11/xorg.conf.d/
10-amdgpu.conf 10-quirks.conf 11-evdev-quirks.conf 40-libinput.conf 70-synaptics.conf
10-evdev.conf 10-radeon.conf 11-evdev-trackpoint.conf 51-synaptics-quirks.conf 70-wacom.conf

Yet my ~/.xprofile gets ignored and xorg uses it's (non existing) defaults.
Checking the currently running session, with the false xorg-defaults, I can not find anything related to a cloning parameter when checking the xrandr and Xorg-logs; so I'm not even sure where this setting comes from.

What I expect:
To get Xorg to either have:
- a configurable working display configuration with external displays and no external displays
- being able to call a custom configuration during an Xsession
- turn off automatic xorg configuration so I can configure it myself

I can reproduce this behaviour reliably.

Thank you a lot!