Hi, please check out “My caustics progressively disappear, there are black spots visible on my rendered caustics!” here: https://support.chaos.com/hc/en-us/articles/4954589213457
…or try our newest daily build where this should be fixed (but please remember that daily builds are experimental and you should make backups of scenes before opening them with daily builds) - https://forum.corona-renderer.com/ Chaos Corona 10 for 3ds Max - Daily Builds Changelog