Anything that runs, or could run, for over 3 hours must, by NEC code, be wired to 125% of the rated maximum load. A 30 amp EVSE requires a 40 amp circuit including wires and breaker.
As to the problem, if it occurred within say 5 minutes its almost certainly voltage drop and is probably a result of the extension cable being inadequate. If it took longer, say 30 minutes, than it's more likely a heat problem causing drop from increased resistance and could be anywhere.
BTW, voltage drop is a function of wire gauge and distance, so the longer you attempt to go the larger gauge wire you need.
Aside from those sometimes faults like this are a result of receptacle lugs not being tight enough. Easy to check right?
I charge at 24 amps and find it imperfect, but acceptable.
The 14-50 receptable FUD is largely repeated heresay. Sure, there are some issues, but any UL listed recpetacle is fine to 50 amps, or 40 amp continuous. At 30 amps its not much of an issue. What tends to happen is people get cheap on wire and then SHTF. Wire is expected to not only carry the current but act as a heat sink for receptacles and breakers Somebody sticks a plain jane 14-50 on 8-3 Romex, runs it at 50a (requires 75f equipment), and the terminations melt. Everybody wants to blame the equipment but most often its loose wire and too small a gauge of wiring. Had it been wired to 60f rated wire, terminated tightly, and run within it's specs any old Leviton or etc should not melt or etc..