Basalt Delta Blocks Override Nether Bricks in Fortresses

The bug

I generated a Basalt Delta Buffet world in Surface generation, and while looking for locations for nice screenshots, I found a nether fortress which was filled to the brim within with basalt, and with some of its nether brick blocks replaced with magma blocks or lava. Nothing like this generates in other biomes, as far as I know, so I am not sure whether it is intentional. Either way, it looks beautiful anyway but I thought I should mention it.

How to reproduce

1. Create a new world with Buffet Generation, whose biome is Basalt Delta and generation is Surface, with cheats on and seed of 5545820836948269667.

2. Type the command /tp @s -326 64 -376

3. Such generation as mentioned above should be observable. Whilst beautiful, I doubt this is intended.