Im not familiar with when Alta might show that page other than when you go to a device that is already configured/assigned to a site. Its possible that might show at other times that im not aware of. Do you have any NAT configured or routing entries configured? Im wondering if this traffic is getting sent to the routers interface via nat rule. If you do a test and lets say your test machine is at 10.0.0.10 address trying to get to 10.50.0.20(test laptop running webserver)
Open a CLI session to the router and see what these give you. You will need to change my example IPs to your IPs. The route should be pointing down the interface you expect to where VLAN 50 lives. The conntrack output might give you an idea if its doing any sort of NAT when you are trying to get to the web page. Run them while you are trying access the page.
ip route show
conntrack -L | grep 10.50.0.20 | grep 10.0.0.10