I have noticed this from a while and had not been able to put a finger on it, I keep subtitles on for most of the movies at night times when kids need to be in bed. I have noticed that I can see subtitles reading but there is no sound or dialogue coming from LRC. I have a 5.1.4 setup on Pioneer VSX-LX303 but had same issue with Sony DH750.
I am guessing that the surrounds should have picked the sound but not sure. Any advice will be much appreciated.