Is this a bug or a "feature"?

In ZDoom mode using Doom Builder 2, I can select both textures and flats for choosing the texture or flat bor both. In other words, I can select textures on flats and flats on walls. Is this supposed to happen?

It is neither a bug nor a "feature", but an actual feature. The ZDoom sourceport (among others) supports flats on walls and texture on floors.