When finding caves under y=0 Water and Lava don't aren't rendered correctly and look like floating carpets, this also may affect Aquifers as they only seem to go one block deep.I also noticed the the texture for stone below y=0 looks brighter than it should than above y=0.
Thank you for your report!
However, this issue is Invalid.
Please put only one bug report in each ticket. It is very difficult to keep track of bugs when they are not in their own tickets. Water and Lava issue is already logged MCPE-126658.
However, please review the Bug Tracker Guidelines before creating new reports. Be sure to search for an existing issue as it is likely to have already been reported.
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