Summary:
Water frozen with the Frost walker enchant (ice) can't be broken by the player. Instead the blocks below/behind the ice are broken or mined or interacted with. (this can be most easily reproduced when the time is set to night, as frost walker created ice will not melt in this condition).

Steps to reproduce:

• (optional) Set the time to night time
• Have a player with the frost_walker (1 or 2) enchant on boots stand over shallow water (for example 1 block deep).
• Try to break the ice created by frost_walker in survival mode or creative

Observed result:
The ice is not broken, but the blocks behind the ice can be broken. (it behaves like water)

Original description:
Place 1 block of water in a hole. Make sure it frozen with frost walker. Leave the game, re-enter the game and try and mine it. You will find you can't.