I am playing with my sister in Survival and I go to swim in water to kill fish and find Drowned. When I do that at night, the water blacks out occasionally. This is not always the case. I am player 2 and it doesn't affect player 1.
How to reproduce:
1. Make sure it is night.
2. Go swimming in any body of water at least 2 blocks deep.
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Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-32799.
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It happens randomly to me as well... Just not at night, usually force closing the Minecraft app and re opening works...