I have my Bitcoin Core client up and running almost constantly but from time to time, when I need more resources, I disable inbound connections by removing the relevant port mapping on my router. It may or may not be relevant that my router does not support UPnP but I have it enabled on the client.
I check if I'm actually reachable with
bitnodes, and it says I'm visible or not depending on the port mapping settings so everything seems to work as expected.
However, even when I am "invisible" to the network, I receive inbound connections. I generally end up with one or two of them, always with a IPv6 address: how is that possible?