I had to reinstall my R7800 after a mishap. It is sitting behind my FIOS G1100.
My home network is 192.168.40.1; the FIOS gateway hands out 192.168.1.x addresses (but only to my openwrt router).
If can access the openwrt from a browser (192.168.40.1) to do setup, but I can also access the FIOS device at the same time at 192.168.1.1.
Is this normal? I don't recall being able to do this before my wipe/reinstall.
To me it looks normal since I assume your client's IP address (192.168.10.y) is translated to 192.168.1.x by openwrt's NAT.
Yes, completely normal. You have double NAT with that configuration though. See if you can put the FIOS device into bridge mode and let the OpenWRT do the routing.
Are you just wondering why it happens, or are you concerned about security?
As people mentioned, it's normal, and in fact required. If you had no route between your PC and FIOS, you wouldn't have access to internet.
If your question was related to security, then it's the other way around that you should check. See if you can have access from FIOS to OpenWrt LAN side. That should be protected by the firewall by default.