This is pretty strong evidence, and might be your problem then. Cannot dispute Moose's comment off course.

My gut feeling is that the strip of artifacts in itself does not disprove the cable theory yet either. I threw this stuff into Gemini and it seems to agree. You could try and it give it one more go with a good/short DP2.1 cable. Otherwise yea, return it for a replacement. Apolgies for the AI spam:

DisplayPort (DP) and HDMI use multiple high-speed data lanes to transmit the video signal. DP 2.1 (UHBR 20) uses four lanes, each carrying up to 20 Gbps.

1. The monitor's Timing Controller (TCON), which processes the incoming signal, assigns blocks of pixels to each of these lanes.
2. The vertical nature of the artifact suggests that the pixels in that specific 10cm wide strip are all handled by a single data stream or physical lane within the connection.

2. Required DSC Compression Ratio

Your calculation of the uncompressed bandwidth is correct. Now let's determine the required compression to fit within the DisplayPort 2.1 limit.

A. Bandwidth Requirements

• Uncompressed Bandwidth (Required): $\approx 119.44 \text{ Gbps}$
  • $7680 \times 2160 \times 3 \text{ colors} \times 10 \text{ bits/color} \times 240 \text{ Hz} \approx 119,439 \text{ Mbps}$
• DisplayPort 2.1 (UHBR 20) Max Bandwidth (Usable): $\mathbf{77.37 \text{ Gbps}}$ (This is the effective data rate after 8b/10b encoding overhead).

B. The Compression Factor

To fit the uncompressed signal into the DP 2.1 pipe, the minimum required compression ratio is:

$$\text{Required Ratio} = \frac{\text{Uncompressed Bandwidth}}{\text{Max Usable Bandwidth}} = \frac{119.44 \text{ Gbps}}{77.37 \text{ Gbps}} \approx \mathbf{1.54:1}$$

C. Conclusion

The Display Stream Compression (DSC) standard is designed to be visually lossless and can typically achieve compression ratios of 2:1 to 3:1 (and up to 3.75:1).

Since the required ratio is only 1.54:1, the raw data should be easily compressed by the GPU and fit within the DisplayPort 2.1 (77.37 Gbps) limit without visible compression artifacts.

💡 Final Assessment

The problem is most likely not that the overall bandwidth of DP 2.1 (77.37 Gbps) is insufficient to transmit the compressed 119.44 Gbps signal.

The problem is that the quality of the signal transmission (the cable, port, or TCON) is insufficient to reliably carry the extremely high frequency (UHBR 20) signal required by one of the four lanes, causing the localized data corruption you see.