I don't whether it's a bug or a feature but on https://en.wikipedia.org/wiki/Help:Displaying_a_formula
I see that
- "\ulcorner \urcorner \llcorner \lrcorner" generates 4 characters, each one in its own <mo> element while
- while "\vartriangleleft \ntriangleleft \vartriangleright \ntriangleright" puts the 4 characters in the same <mo> element.
I'm not sure this situation happens in "real" math, though.