This is not true.
It would be things encoded in the center back that disappear in mono play. This happens in both QS and SQ. In fact, almost all matrix systems (including Dolby Surround) have the center back encoded so it disappears in mono play. Only BMX, G, H, HJ, and UHJ (Ambisonic) do not do this.
Things encoded in the front center in both QS and SQ are louder than other parts in mono then they are in stereo or matrix playback.
When things disappear from the center front is if you are playing them in Pro Logic, you don't have a center speaker, and the speaker mode is set wrong in your decoder.
Maybe it was background vocals that disappeared in mono play. Those are often mixed in LB and RB.
H gets rid of the center front and center back problems but puts phasiness in stereo playback instead.
HJ and UHJ mostly remove the phasiness problem, but the center front is louder in mono again.