[NAME]  The MST bits are MST chipsets within the dock because the system sends two DisplayPort interfaces within the Thunderbolt connection, and the dock also needs to be able to split those out across its three output ports.  But the fact that the dock has an MST chip internally to split a signal across its own outputs does not mean it can pass an MST signal through its DisplayPort outputs to run a daisy chain "downstream" of its outputs.  I completely understand the point of MST as I use daisy chain setups with systems and docks that support them, but like I said, I don't think the TB16 is one of them.  Mirrored output as you're seeing now is what you get when you have a correctly cabled daisy chain connected to a device that doesn't support daisy chaining.
okay, I have my brand-new shiny Dell XPS 13 9310 and a brand-new TB16 Dell Dock and my old trusty 2516 monitors (×2) I have been running my existing PCs through daisy chain MST on these monitors  for years now, only slight issue was blanking screen every so often which was fixed by running full-size DP to mini DP (in that order) through the system. So basically, been working perfectly for years. I get a brand-new XPS 13 and look forward tremendously to extending my new acquisition for work from home through my setup… Plug it in… The best I can get is the same screen 3 times and it will not detect the other 2 monitors (as one of them is in landscape mode, this is a big problem anyway). I have updated the drivers in everything, the cabling is all pristine and works with everything apart from the XPS 13. Can anybody help me out with a solution to this?
XPS 13 & TB16 dock won't daisychain properly to 2× UP2516D Dell monitors