I have had a T420 on dock with which I had no problems - the difference though was that I used the DisplayPorts on the dock.
The system monitor setup has been working - so what have changed?
If you boot a live Manjaro Budgie ISO - what happens?
Does it work or does it not.
Let us the result of the live ISO boot.
If it does you need to compare the configuration of live with your running system.
Is it a periodic issue - I mean you have waited several months before reporting and what have you tried besides the dockd package?
It would also be valuable if you could peek the config of Ubuntu Budgie and compare the config to your running system.
Another thought is - Ubuntu is not known for being up-to-date - package wise as it is a fixed release.
This leads to thinking that Ubuntu Budgie packages lags behind Manjaro and maybe an issue exist with later Budgie packages which has not yet reached Ubuntu but Manjaro.
As I don’t know much about Budgie I have to ask if the Ubuntu Budgie version the official Solus version or it is an Ubuntu offspring.
EDIT: I found according to Wikipedia that Ubuntu Budgie is an official Ubuntu flavor.
On wikipedia it states that version 11 of budgie will be rewritten in Qt and start using Wayland.
I have no idea of where Manjaro Budgie is in this regard. Is it v10.x?
In that case it might be an idea to test Manjaro Gnome - as it seems to be close to Budgie. But I see that @muvvenby already suggest something similar.
My Thinkpad - as I recall had no Nvidia card - just Intel.
Could you please post your
inxi -CGMAxx preferably when attached to dock.