XPS 9510 - Inconsistent TB/USB-C Charging Behaviour

I've got a new XPS 9510, which seems to be exhibiting some very odd behaviour around what it will / won't charge from.

• The included power supply works fine. (Diagnostics shows 120W)
• D3000 USB-C docks work fine (albeit at lower wattage - I think this showed 65W in diagnostics)
• WD15 USB-C/DP docks work fine (albeit at lower wattage - I think this showed 65W in diagnostics)
• Belkin TB3 Express Dock HD doesn't really charge (Diagnostics shows 15W - presumably this is just basic USB rate charging as opposed to PD / TB)
• Samsung UW Screen (Not sure what model)  doesn't really charge (Diagnostics shows 15W - presumably this is just basic USB rate charging as opposed to PD / TB)
• LG Ultrafine 5K Screen 
•   doesn't really charge (Diagnostics shows 15W - presumably this is just basic USB rate charging as opposed to PD / TB)

Was hoping to use my screens to dock it, or to buy a TS4 or similar. It however seems that the device can't charge itself from TB. Ideally i'd have a WD19TBS to try it with, but would much rather buy the (more capable) TS4. 

Whats more frustrating is that if its receiving power from the screen or dock, it won't accept power from its own power adaptor plugged in to another port - Diagnostics shows 15W, and it seems to have picked the TB charger instead. If I could get it to pick its own adapter when using my screens, I'd not be fully happy but it would be a workaround.

Is something broken? I keep going around in circles with Dell support as its not running Windows, but presumably that shouldn't make any difference? The devices are showing 15W in Bios diagnostics.