I have encountered this countless times on realms.

When falling from a height into water, while wearing Frost Walker boots, a layer of ice forms, either underneath the player causing fall damage, or above the player causing them to be trapped underwater underneath the ice. The difference between those two may be connection speed but I am not sure. I have a slower connection and usually I get trapped underneath the ice.

On Java Edition (as far as I know), Frost Walker only produces ice when the player is actually walking across a block at the same y level. It does not produce ice when falling into the water, which is good because it does not produce the weird and inconvenient results that occur on Bedrock Edition. The Bedrock physics could be improved by bringing them into parity with Java in this way.