So the electrician failing to properly terminate one wire in an Ethernet jack turned out to be my brown M&M. Since I realized he didn't test the jack I also no longer trusted him with power outlets and bought an outlet checker on Amazon
Sure enough, one outlet is literally upside down (but at least wired correctly), and two outlets have the live and neutral swapped…
#Japan requires an electricians license to as much as touch a power outlet, because “DIY people can't be trusted with safety…”