It was discovered during survival map play, the water was blocked by the lichen, so the water was floating right above that like ladders. and when I fell under the water, I fell directly to the floor and got damaged.
After the discovery of this phenomenon, I experimented with lava on a different map, and it was the same result.
Environment
Windows: Windows 10 Pro
Java Version: 1.8.0_51 64bit
CPU: 6x AMD Ryzen 5 3500x 6-Core Processor
GPU: NVIDIA GeForce GTX 1660 OC
Duplicate of MC-212116, which was later resolved as, "Works As Intended". Please see the parent ticket for further details.