Acrylic gesso's opaque pigments

I cannot speak for manufactures, but I do agree with you that there would seem to be little reason to include zinc white in a ground intended to be opaque as well as white if it is bound in an acrylic dispersion binder.  Zinc was traditionally added too or as the sole white pigment in color mixtures and tints in oil paint since titanium white make a weak paint film. We now know how damaging the presence of zinc white is in oil paint (although was have know of its brittleness for a century or so) It does not have this deficit in acrylic dispersion paints but as you wrote, it would seem unnecessary and even counterproductive in a white acrylic dispersion ground.
Whoever, I am not an expert in the production or even use of acrylic dispersion paints and grounds. My background as an artist and later my focus as researcher and conservator is in oil paint, egg tempera, other natural aqueous paints, and to a lesser degree historical drawing materials. There may be reason that I do not know about why a manufacturer would use some zinc white in a ground. I will ask another moderator more well versed in acrylic dispersion paint to comment.
The last part of your question concerns the idea that if there is some zinc oxide in a ground, perhaps some of the oil absorbed from superimposed oil paint might react with the zinc and confer the deleterious effects to the oil paint. Perhaps the zinc oxide particles would be surrounded by the acrylic dispersion medium and would, therefore would be isolated from the fatty acids in the oil.  However, we do know that there is far more movement of fatty acids and other compounds through different paint layers so perhaps there is a concern about this. I do not know the answer and much testing would need to be done to know the answer for sure. I will send this to a couple of moderators who can perhaps be able to speak about this with more authority.

​In GOLDEN Acrylic Gesso there is only Titanium White for color and opacity and Calcium Carbonate for absorbency and tooth. On pre-primed canvases with oil grounds, zinc is added because it reduces dark yellowing. That is a concern when a bale of pre-primed canvas is rolled up for storage. In acrylic dispersion grounds there is no point in adding zinc, although we have not analyzed other manufacturer’s gessos. We are currently testing whether oil applications over acrylic zinc paint causes embrittlement as well. 

​We understand that it is possible some manufacturers may add zinc oxide to acrylic grounds. We do not see any reason to add zinc oxide to acrylic grounds, although almost all oil-primed canvases sold today contain zinc oxide in their oil grounds.
Some recipes I’ve encoutered for traditional chalk and gesso grounds contain zinc oxid, espec ially those used for silverpoint. The belief is that zinc oxide enhances mark making with silverpoint, but this has not been proven by tests.

​Utrecht brand Acrylic Gesso does not include zinc, just titanium white and calcium carbonate.

Thanks for the info​