Well, my AXE16000 seems to work as you would expect. When I am connected to the Guest network, no devices on my local network are shown in File Explorer-Network devices.
I will say, when an AiMesh configuration is involved, it might get complicated. There is a setting for allowing the Guest network to one or more Mesh Nodes, so perhaps that setting would help.
Edit: This was before the firmware I just got. After the Update, the Guest Network still appears to work. But when the update was finished, I got a message I had entered an incorrect password 5 times and could not log back in until after a time break. I had not tried to enter a password during this period.
Maximus Z790 Hero,