What’s DDC?

My symptoms included:

• The monitor being set a screwy resolution when first connecting to the computer (but allowing the proper resolution after tweaking in macOS or Windows display settings)
• The monitor’s manufacturer and model information not being picked up by the computer
• DDC/CI commands being completely unavailable to any connected computer’s software, making software-driven brightness control unavailable

For me, it was because I was using a HDMI->DisplayPort cable, and that in-cable adapter must have had some shit logic in it.

It was all fixed by using a different cable to connect the computer to the monitor.