Action icons do not appear when the game is using an external GPU

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Date & Time: irrelevant
Frequency: always
World name: irrelevant
Character name: irrelevant
Retainer name: irrelevant
NPC name: irrelevant
Monster name: irrelevant
Class/Level: irrelevant
Party or solo: irrelevant
In-game time: irrelevant
Area and coordinates: irrelevant
Housing: no
Steps:
1. At the Finder, without the game running, connect an external GPU via Thunderbolt 3 to a Mac (I used a Sonnet Breakaway Box)
2. Open Final Fantasy XIV
3. Sign in
4. Select a character
5. After login is complete, open Actions & Traits, or look at the hotbar

I expect that there would be icons in Actions & Traits, as well as the hotbar, but there are transparent squares where almost all of the icons should be (a few may show up). I can provide screen shots, if necessary.

The icons are also not present in the system configuration window.

The problem only happens when the computer is connected to an external GPU. It does not happen when playing the game using the internal GPU. I don't know if it's a problem with this specific GPU, or if it's a problem with external GPUs in general.

Connection Specs
- Type of internet connection/provider: irrelevant
- Modem maker/model number: irrelevant

■ PC Specs

To obtain the system information, open the FFXIV launcher and select Config → System Information → Display. (I would, but that option appears to be missing from the macOS client. So I'll copy & paste from System Information instead.)

-=-=-=-=- System Information -=-=-=-=-
Hardware Overview:

Model Name: MacBook Pro
Model Identifier: MacBookPro13,3
Processor Name: Intel Core i7
Processor Speed: 2.9 GHz
Number of Processors: 1
Total Number of Cores: 4
L2 Cache (per Core): 256 KB
L3 Cache: 8 MB
Memory: 16 GB
Boot ROM Version: 250.0.0.0.0
SMC Version (system): 2.38f7

Radeon RX Vega 64:

Chipset Model: Radeon RX Vega 64
Type: External GPU
Bus: PCIe
PCIe Lane Width: x16
VRAM (Dynamic, Max): 8176 MB
Vendor: AMD (0x1002)
Device ID: 0x687f
Revision ID: 0x00c1
Automatic Graphics Switching: Supported
gMux Version: 4.0.29 [3.2.8]
Metal: Supported, feature set macOS GPUFamily2 v1
Displays:
XG2700 SERIES:
Resolution: 3840 x 2160 (2160p 4K UHD - Ultra High Definition)
UI Looks like: 1920 x 1080 @ 60 Hz
Framebuffer Depth: 30-Bit Color (ARGB2101010)
Main Display: Yes
Mirror: Off
Online: Yes
Rotation: Supported
Automatically Adjust Brightness: No


-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-


Laptop Model Name: MacBook Pro
Laptop Model Number: MacBookPro13,3

Gamepad Manufacturer: irrelevant
Gamepad Model Name: irrelevant
Gamepad Part Number: irrelevant

Browser Information
- Name and version of the browser currently in-use: Safari 12.0.1
- Internet Explorer Version: there is no Internet Explorer on macOS