Hello,
i just got asus ROG STRIX Z390-I Gaming and manual how to install ram is completely incorrect. It shows to put ram straight into socket, not in a angle, like in a angle 0. In a straight line, you know what i mean... And actually you are supposed to install them by putting them in angle like 30 degrees with longer side first. Which is completely different than what is in manual. So you may want to rewrite your manual...

Look at this https://www.youtube.com/watch?v=e9fQE6mssvg&t=154s video this is the correct way how to install ram to this motherboard and now look at your manual. You can't put ram straight in, because they are longer then socket. They have to be put by angle in very specific way. It makes sense to they don't fall out, after mounted...