The barrier block should be invisible, unless the barrier item is held; in that case, the block should be a red ø that swivels to face the player.

That's not happening (see attached picture ending in .19, and note the invisible block in the crosshairs surrounded by black lines to indicate that there's actually a block there).

I have tried placing barrier blocks on several types of blocks, so it would seem to be an issue with the barrier block itself, rather than an issue of placing it on the nether brick fence.

The map in which I took these pictures and in which I have subsequently been testing this bug was created in 1.12.

Just out of curiosity, I went back to my 1.8 map to see if the barrier blocks in there worked as intended; they didn't.

Otherwise, the block functions normally: entities can't pass through it; blocks can be placed on it; when broken, the block does disperse red particles like it would normally (see attached picture ending in .21), and it is impervious to being blown up.