Why has no one asked the wattage of the heater?
Electric space heaters often, mostly, use nichrome wire for the heating element. Nichrome has a very HIGH positive temperature resistance co-efficient. Low resistance at room temperature that rises dramatically as the element heats.
A 1000 watt heater might draw 3000-5000 watts during the first few millisecond after the thermostat closes. Over time the heat from that effect will result in the loss of the receptacle contact gripping, springiness.
MH builders are cost-conscious in the EXTREME, CHEAPEST components that are in the market, generally.