I’m wondering if:

1) First set of cleats, from Giant store (so highly likely to be genuine), was simply a matter of too much walking and stairs, as you suggest. I didn’t know better.
2) The second set I actually got from a friend, and were in Shimano packaging, but I don’t know their source. So perhaps fake?

My third set, currently in use and early days, have already shown far less wear than the other two. These are genuine Shimanos (reputable store, authentic packaging).

Time will tell!