I noticed my Center speaker, and hence dialogue in videos, appeared much quieter some time ago. I put it down to a replacement Center speaker, ageing ears and some hearing loss. Maybe there has been a change in MC, because a few people have mentioned quieter dialogue.
In fact for video playback I boosted my Center speaker 18dB, plus played with some specific frequency volume changes. I'm still playing with that.
I output 5.1 in Dolby Digital format over a Toslink cable, so my configuration is probably somewhat different to yours. But I did some testing anyway.
Yes, the "Move center channel to L/R" is working properly. I did both a hearing test and watched the Analyser, both of which showed the Center had zero dB output. The left and right were both boosted in volume a bit. I assume that function actually adds the Center to both the Left and Right, since a Move would replace the original L&R outputs. Maybe it does some volume adjustments as well, so as not to overdrive the L&R channels.
Maybe you should boost the L&R channels a bit using the Parametric Equaliser to compensate. It would be better if you could boost the Center channel before moving it, but unfortunately the "Move center channel to L/R" function is part of the "Output Format" function, and that must happen first in the DSP, so you can't use the Parametric Equaliser to boost just the Center channel, since it isn't there any longer.