A barrier block placed underwater cannot be viewed by a player above the water, even while said player is holding a barrier in their hand.
Similarly, a barrier block placed above water cannot be viewed by a player underwater, even if the player is holding a barrier in their hand.
Bug does not exist when "Fabulous" graphics on.
Rendering issues like this are only accepted for Fabulous graphics setting; it's specifically added to fix these kinds of bugs. This is fixed in fabulous.