the only thing I can think of is that the Mageia grub video driver is not compatible with your video card.
(and I do not know how to change it)
I am only going on, that, from ages ago, my original unichrome card would sometime not display random text, letters / button labels...etc, and changing the card was the only way I could fix it and during the testing phase of Mga7, it was also discovered that the installer was "picky" about the intel 810 and later driver - missing buttons / text / graphics so a change was made to the installer graphics to overcome this.
so video card is?