Two thoughts. First, do your firewall rules allow the port forwarding? If you have an "all all DST-NAT" rule in the forward chain, that would take care of it, but if you don't do that, you generally need to specifically allow the forward. This is not likely the case since port forwarding worked on port 8081, so you either have the first allow rule or you added a specific one.
Second, could your ISP be blocking port 8080? You could add a counter rule early in your forward chain to make sure that packets are actually arriving at the router. Those can be very helpful in troubleshooting. Here is an example of one of mine:
add action=passthrough chain=forward comment=\
"Counter for inbound UDP port 32222 packets to Black Forest GRIL" \
connection-state="" dst-port=32222 in-interface=E1-p2_Cable_Internet \
Obviously, that rule needs to be before any other rule that would allow the packet (such as an "allow all established and related" packets).