The thing is: TB docks are _very_ complicated devices, consisting of multiple subsystems each running their own firmware. With the Lenovo dock (and I assume the Dell's are not much different) there have been numerous firmware updates over the past years. Updating them usually requires a windows application, the Linux fwupd can only do some (but not all!) firmware updates for the dock.

Anyway, what I am trying to explain here is that you should check if you have updated all parts of the dock's firmware. I recommend to use the Windows tools for that, don't trust fwupd to be able to update all parts of the dock.

I also suffered from the screen randomly going black, but that turned out to be a firmware issue with my display (LG 43" 4k). As it turns out: firmware on a display can also be updated. I needed to connect my laptop through a USB-C cable to the monitor, then run an update tool from LG. The update tool was of course windows only, and even buggier than the Lenovo dock firmware updater, but it did work! No more screen blackouts.