Forced mirroring on two external monitors with Lenovo ThinkPad Thunderbolt 3 Dock

When connecting two external displays (both Dell U2715H) using USB-C through a Lenovo ThinkPad Thunderbolt 3 Dock they always come up with the same image. So the external Dell displays are mirrored while the built-in Macbook Pro Retina with Touchbar display shows its own part of the extended desktop.

When using system profiler, only one of the external displays is listed, although both displays are showing (the same) image. The Display preferences pane only shows one external monitor and that mirroring is off.

I have installed the DisplayLink drivers. The monitors have both been reconfigured to run DisplayPort v1.2.

It doesn't seem to matter which cables are used for the displays, or if they are both using the same connection or e.g. one connected with DisplayPort and the other one with HDMI. Each is plugged into its own monitor channel on the back of the dock. The Macbook Pro is plugged into the USB-C port on the back (the one marked with a computer symbol).

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$ system_profiler SPDisplaysDataType

Graphics/Displays:

Intel HD Graphics 530:

Chipset Model: Intel HD Graphics 530

Type: GPU

Bus: Built-In

VRAM (Dynamic, Max): 1536 MB

Vendor: Intel (0x8086)

Device ID: 0x191b

Revision ID: 0x0006

Automatic Graphics Switching: Supported

gMux Version: 4.0.29 [3.2.8]

Metal: Supported

Radeon Pro 460:

Chipset Model: AMD Radeon Pro 460

Type: GPU

Bus: PCIe

PCIe Lane Width: x8

VRAM (Total): 4096 MB

Vendor: ATI (0x1002)

Device ID: 0x67ef

Revision ID: 0x00c0

ROM Revision: 113-C980AF-908

Automatic Graphics Switching: Supported

gMux Version: 4.0.29 [3.2.8]

EFI Driver Version: 01.00.908

Metal: Supported

Displays:

Color LCD:

Display Type: Built-In Retina LCD

Resolution: 2880 x 1800 Retina

Pixel Depth: 32-Bit Color (ARGB8888)

Main Display: Yes

Mirror: Off

Online: Yes

Rotation: Supported

Automatically Adjust Brightness: Yes

Connection Type: DisplayPort

DELL U2715H:

Resolution: 2560 x 1440 @ 59 Hz

Pixel Depth: 30-Bit Color (ARGB2101010)

Display Serial Number: GH85D5[redacted]

Mirror: Off

Online: Yes

Rotation: Supported

Automatically Adjust Brightness: No

Connection Type: DVI or HDMI

Adapter Firmware Version: 3.0c