I have the exact same problem with my 2019 16" MacBook Pro. My Windows computer works great with the monitor using the monitor's native resolution (5120x1440 @ 120Hz), but the resolution on my 16" MacBook Pro does not allow the native resolution and only lists 3840x1080 (by clicking with the "Option" key on the Scaled Resolution selection in the Display properties).

I have installed SwitchResX which is setup to use the native display resolution , but the it seems a bit blurry and flickers sometimes.

The monitor drivers pointed out in the prior message are for Windows only and my macOS is fully up to date (10.15.4)
Based on my hardware details, my MacBook Pro should handle up to two 6K displays (6016x3384), why does the resolution not work natively in macOS with this monitor?
I am using a full TB3 cable to connect directly, and again, my Windows 10 computer works great with the proper resolution.
Can anyone help? Can I use my Joint Venture support or my company Apple relationship to speed up this resolution in some way?
Here are the Graphics/Display details from the System Information app:
AMD Radeon Pro 5300M:
Chipset Model: AMD Radeon Pro 5300M
Type: GPU
Bus: PCIe
PCIe Lane Width: x8
VRAM (Total): 4 GB
Vendor: AMD (0x1002)
Device ID: 0x7340
Revision ID: 0x0043
ROM Revision: 113-D3220E-190
VBIOS Version: 113-D32207P1-019
Option ROM Version: 113-D32207P1-019
EFI Driver Version: 01.01.190
Automatic Graphics Switching: Supported
gMux Version: 5.0.0
Metal: Supported, feature set macOS GPUFamily2 v1
Displays:
Color LCD:
Display Type: Built-In Retina LCD
Resolution: 3072 x 1920 Retina
Framebuffer Depth: 30-Bit Color (ARGB2101010)
Mirror: Off
Online: Yes
Automatically Adjust Brightness: No
Connection Type: Internal
SwitchResX4 - AG493UG7R4:
Resolution: 5120 x 1440
UI Looks like: 5120 x 1440 @ 120 Hz
Framebuffer Depth: 30-Bit Color (ARGB2101010)
Main Display: Yes
Mirror: Off
Online: Yes
Rotation: Supported
Automatically Adjust Brightness: No
Connection Type: Thunderbolt/DisplayPort
Television: Yes