2 of 3 screens have same background in Windows 10 shuffle

My setup:

• Dell XPS 8500 Special Edition, A14 BIOS
• CPU: Intel Core i7-3770
• GPU: AMD Radeon HD 7870 GHz Edition, fully updated drivers (v. 18.12.2)
• OS: Windows 10 Pro v1803 64-bit, fully patched and updated
• 3 monitors, connected to Radeon as follows:
  • Dell U2713HM - 1440p - DP - 60 Hz
  • NEC EA275UHD - 4K - DP - 60 Hz
  • ASUS PA248Q - 1200p - HDMI or DVI-D - 60 Hz

Steps to reproduce:

1. Connect the monitors as above
2. Set wallpaper background to shuffle in Windows Settings

What happens:

2 of the monitors (always adjacent to each other) have the same background.

What should happen:

All 3 monitors should have different backgrounds.

Anyone ever seen this before? Any ideas?