DIAGNOSIS: That's the local dimming algorithm struggling with fast-moving, high-contrast content. In Game Mode, the TV reduces processing to lower input lag, which can make the dimming zones more aggressive and visible, causing "blooming" or halos around bright objects.
FIX: You can't turn off local dimming entirely on this model, but you can change its setting. Press the gear button on the remote to open Quick Settings, go to Picture Settings, then Advanced Settings. Find "Local Dimming." Try switching it from "High" to "Medium" or "Low." This reduces the intensity of the effect and can minimize the blotchiness, though you'll get slightly less perfect blacks.
IF THAT DIDN'T WORK: For gaming specifically, if the blotchiness is still bothersome, you might have to choose between low input lag and perfect picture. Try switching out of Game Mode to a different picture mode (like "Movie") which uses a more refined, but slower, dimming algorithm. You'll get a cleaner picture but higher input lag.
HEADS UP: This is a limitation of having a finite number of dimming zones (this TV has hundreds, not thousands). Very fine, bright highlights on dark backgrounds will always show some blooming—it's the trade-off for the high contrast ratio this TV delivers.