This is probably more of an "important feature request" rather than a "bug". I just didn't know how to properly mark it as such. Anyway:

If barriers are always invisible, I can imagine it would make it slightly more difficult/cumbersome to see what you have done as a map maker, especially when debugging your recently used block fill commands during experimentation.

I think It would therefore be helpful and aide map making if the barrier blocks in the world would show as visible blocks, or simply the red circle+line symbol. This would then only be the case when 1) in creative mode (which I assume is the only place where you will be able to get a hold of the barrier blocks) and 2) holding a barrier block in your hand. The reason for requiring it to be in your hand would be because, firstly, most people probably place it by hand instead of using block fill commands and, secondly, because when placing blocks in the world during regular map making you might not want to see barrier blocks, but instead what the world will look like to the "adventurer", instead.

Hopefully Mojang and players will think this is a good idea and proper user interaction for this neat feature. Greetings from Sweden!