I have done a bunch more testing and I am 99% sure the touch sensitivity issues have to do with the iPad being electrically grounded. If you touch it in any way with your opposite hand, or have it plugged in, it works perfectly in any orientation. As soon as it's just sitting there by itself and you try to type on it (flat on a table or propped up in a case, for example) all touch functionality is removed. If you push aggressively with the tip of your fingernail, you can get some touches to register in a pinch.
Now that I've figured it out, I have no idea what the solution is.
Got another refund (7th or so) from the last glass screen protector I tried. I guess I'll just have to live with ALWAYS having the iPad grounded (either by my other hand touching it, plugged in, etc.) or I simply can't use it if I want to avoid scratching the screen. Kind of takes the shine off a $1200 tablet. Oh well.
Those of you that claim it's working even if nothing is touching/grounding the iPad, I'd love to see a video showing 100% flawless touch response propped up in a case or flat on the table. I've been through every brand of glass so far including now 2 JetTechs (literally flawless installs, zero bubbles, not one speck of dust) and the Face ID always works just fine but touch response is 100% removed unless the above conditions are met.