I use VLANs as well and the only place where I see the active DHCP leases for the Guest or other LANs that I’ve created is on the Overview and the Network/DHCP/Static Leases page(s) of the Advanced LUCI GUI.
It would be nice to at least see the Guest network leases in the GL GUI Clients page since the Guest network subnet can be configured in the GL GUI.
Can you see you Lan clients in the gui now you have changed that to br-lan.10 ? Usually the gui is hard coded to only see two bridges and that's where the problem lies.
In order to see your vlan clients (which would only work for one vlan) is to create a br-guest bridge, as that is what the gui is hardcoded to see.
So what you need to do is create the vlan, in your case you create vlan-lan.99, you may even need to create the vlan off the actual port for example eth0.99 and not from the LAN itself either way you need will need to test.
So create the vlan then go back and create a bridge device, call it: br-guest then for the bridge ports select your newly created vlan and now on your interfaces page you should see guest showing as br-guest and not br-lan.99 and that is the key to get the clients in the gui. This will only work for one vlan as we are just using the br-guest to get them in gui. I'm not sure if this will effect your guest network, it shouldn't I don't think but I set mine up this way on the brume series as they don't actually have a guest WiFi as there is no WiFi.
But again, the trick here is to create a bridge called br-guest and then assign your vlan to that bridge.
Devices will then appear in your main gui and you can apply the same rules (limit, block) as you can on your main LAN clients.