Sounds like you should have replaced some pipes as well as the valves. If you have galvanized plumbing, it's only a matter of time before you have to replace it.

There is very likely a piece of galvanized steel supply line very close to the faucet.

There shouldn't be any steel anywhere in there. There supposedly was no plumbing replaced except the valves, and those were all Grohe. I guess I need to check with the plumber and ask if they put any steel in there.
Thanks all.