Re: why wouldn't normal FCC bandwidth and modulation tests NOT pick this up?
"why wouldn't normal FCC bandwidth and modulation tests NOT pick this up?"
Because they are about interference to other equipment and susceptibility to interference.
You can make tranceivers that work for 100s of km that meet FCC limits.
Power line ethernet isn't powerline based at all. It's actually mostly RF. It shouldn't be legal anywhere, it's not properly tested.
FCC, CSA, TUV, CE etc testing doesn't check for stuff like this. Quite a lot of emission is legal and the content isn't checked. Mostly there are only military standards for security. It's entirely optional in consumer gear, though GDPR might change that.