I don't call them "maul-the-band" compressors for nothing...

Haha okay, so would a normal compressor be better?

It sounds amazing when its not mauling the bands.

Any idea on how I can fix this?

Remove it - if it's making the entire mix duck then you've got it inserted in the wrong place.

when the singer ups his volume for a phrase or line

It's your job as the engineer to even that out. Ride the faders, as it were...

Your mix should sound amazing without a comp on the stereo bus.

I use multiband compressors only as a last resort, and even then, very sparingly. I do a bounce/mix and compare it to the the original without it. (Putting them both on a timeline and toggling back and forth between them to hear any difference.) This way I know if I'm doing more harm than good.

My own philosophy (esp with today's DAWs capabilities) is to make sure each track or groups of tracks is ALREADY behaving the way I want them to before mixdown. You CAN, for example, gently duck the backing track(s) in various groups behind the vocal in the mixdown (as long as it's nothing too radical and reversible in case you don't like it later), and you can do the opposite for the vocal - perhaps a little boost here, or a little nip/tuck there, instead of trying to do it all - to the entire track - at the mastering stage.

Many producers have the vocal completely tamed by mixdown time anyway - comping the best takes, de-essing where necesary, a little limiting, etc. (to the vocal, not the music, etc.) and whatever else is needed to create a good vocal track. "Fixing" the vocal track at the end - at the expense of everything else in the stereo mix - is just barbaric and not needed anymore in today's DAW-driven world.

Yeah, it really sounds like you are mashing your entire track, through an improperly adjusted multiband limiter? I mean anything that's more than one must be better? Shouldn't it? Not really. Multiband compression is useful in only a handful of situations. The deleterious effects that you mention are largely based upon attack and release times, along with improper ratios/slopes/crests. I really haven't heard many multiband compressors I like. I much prefer mostly wideband compressors/limiters where I can adjust the bandwidth only in the detector circuit/algorithm. This kind of wideband compression I find more musical sounding as you can make the compressor/limiter only work on certain frequencies without affecting others. This is only applicable to a compressor plug-in that offers more in-depth tweaking aside from just ratio, attack, release.

10 pound bag of stuff in a 5 pound bag
Ms. Remy Ann David