Some women do wear their belt a bit higher on the hips. Wearing it at the waist is somewhat rare and I mostly see it at lower ranks. What I usually see is top of the hips:
But your struggle is real! Lots of BJJ players, particularly women, fight "upward belt creep". The jacket falling open is totally normal. The natural state of a gi jacket is half-tucked, with a belt half-tied. This women's gi BJJ highlight has maybe 50% of jackets open at any given moment.
I feel your pain. For some reason, I too have trouble keeping my gi jackets from falling open within seconds of rolling, drilling, or even warming up. (The only ones that don't are the ones that have a long "skirt", such as a slightly larger double-weave gi that I wear for judo competition.) There is even a small minority of male BJJ players who wear ludicrously short gi jackets in class, and prefer to tie their belts low on their hips, leaving only a few centimeters of jacket tucked below the belt. Naturally this comes loose with the first grip taken. To me this is extreme, and I notice that women in BJJ tend to be less likely to follow it. But it goes to show just how unusual it is for a gi jacket to stay in place in BJJ.
The solution is to get good at re-tying your belt fast, at re-tucking the jacket between rounds, to be OK with your jacket flopping about, and to tie it wherever feels best. I find that tying the knot super-tight (and softening the belt so it can be tied so tight) helps keep things together as it slides around.
