This is a known issue on devices with Snapdragon 855 and older Snapdragon SoCs running Android 10, for instance the Pixel 4, OnePlus 7 and OnePlus 7T.
The flickering is due to a bug in the graphics driver for the Adreno GPU in these devices. Since you cannot update the graphics driver separately, there is no solution other than to wait for an OS update from the manufacturer of your device. For example, the Pixel 4 has this issue on Android 10, but it runs correctly when the device is updated to Android 11.
If you see flickering when running 3DMark Wild Life, please check if there is an Android 11 update available for your model. It's likely that all manufacturers will include an updated driver with their Android 11 updates.
The issue is purely cosmetic and does not affect your benchmark score. Despite the flickering, the test still performs the required rendering work on the affected devices. You can use your Wild Life scores to compare devices.