Suspend not working if power is plugged in

For quite a while now I am dealing with the issue that my laptop running Linux won't suspend when power is plugged in.
Things will work as expected when on battery, but whenever power is added through cable or my docking station, the machine will suspend but immedeatly come back out of sleep.

Suspend has been working after I received the laptop beginning of 2018 but at some point in time things changed.
Since then I have changed Linux distributions, upgraded the BIOS, went through hoops and leaps on forums but no success.

Right now I am running Arch, but I can reproduce the issue running vanilla Ubuntu 19.10 booted from a USB stick, which is why I hope to get some help here.

I tried "systemctl suspend" with...
- lot's of different kernels, recent and LTS
- set all S3 entries in /proc/acpi/wakeup to disabled
- turned off TLP power management
- added kernel boot parameter acpi_rev_override=1-5, with and w/o acpi_osi=!
- checked BIOS parameters, all wake-on-* settings are set to off
- disabled hyperthreading.
- no errors in /sys/kernel/debug/suspend_stats
- no (obvious) errors in journalctl
- different video drivers, intel, nVidia, Bumblebee
- shutdown Network manager before systemctrl suspend
- different OSs, Arch (installed), Manjaro (USB), Ubuntu (USB)

Over time I opened threads here https://forum.endeavouros.com/t/suspend-hibernation-laptop-wakes-up-again-immedeatly-if-power-is-connected/879 and here https://bbs.archlinux.org/viewtopic.php?pid=1874400#p1874400 and people were knowledable and helpful but eventually ran out of ideas.
So my last hope is that this is maybe a hardware/BIOS issue which this community or the vendor know about?

Thanks