If you get the parameter definition of an envelope LoopStart parameter, its default value returns as 1.
However if you try and set the parameter to that value you get an error.
Most parameters can be reset with the paramDef.default value, but not LoopStart though.
zone = this.parent:getZone()
def = zone:getParameterDefinition('Amp Env.LoopStart')
zone:setParameter(def.name, def.default)
raises this error : Error: Line 3: The value must be greater than 1 and smaller than the number of envelope points 4: zone:setParameter(def.name, def.default)