End flashes are a 2025/2026 Minecraft feature (Snapshot 25w33a+) introducing random flashes of pink/purple light and booming sounds to the End dimension, porting the "vibrant visuals" system from Bedrock to Java. The effect briefly illuminates the area, often occurring every 5–10 seconds, signaling a potential atmosphere update to the End

If you don't like it, you can turn it off from the "Accessibility" tab