"Different nodes will have different ideas of what is known and what is unknown so it may be learned unicast on the sending node but unknown unicast on a middle node."

Yes, it's easy to forget that different hosts have different perspective of the network at different times.

Thanks for the image, it's very helpful indeed! I totally understand your point now.

At one time I used to work much much more with these types of details and now work up further in the stack. There is still lots of bridging but its all virtual bridge interfaces on Docker container networks and so things like this dicussion hardly come up anymore for me. This discussion really helped jog my memory and realize I've forgotten some things but also made me realize how fun and fascinating networking is! Cheers!