Since the 2013 MY, the portable EVSE supplied with the Leaf apparently has in its 15a plug a temperature sensor mechanism to help guard against fires caused by excessive overheating occurring in the vicinity of that plug. I say this because beginning in 2013 (and thru 2017) this EVSE has an additional 7th itemized fault condition mentioned in the Owner's Manual with the following description: "When the temperature of the electrical plug is too hot, or the EVSE is unable to detect the temperature of the electrical plug, check that the electrical plug is connected correctly.".
It makes sense that the new 120v/240v portable EVSE has AT LEAST this degree of safety built into the 240v plug and perhaps also duplicated in some way in the 120v adapter -- which probably accounts for their larger than normal size. This is reasonably at least part of why Nissan recommends AGAINST using either an extension cord or another adapter, as either would eliminate or partially eliminate this built-in safety feature.
I checked today, but could not yet find a 2018 Owner's Manual via usual online URLs: "https://owners.nissanusa.com/content/te ... manual.pdf