I do partial boil partial mash biab. I do like 5-6lbs of grain in 2 gallons of local spring water. I spread out my winter hunting coat with two pot holders on the inside back of the coat to set the BK/MT on. pull the hood over the lid,wrap the sides around,& tie thwe sleeves once. Last Sunday I didn't tie them very tight,& the temp went down from 152148F just like yours.
Then last Wednesday,makin another PM IPA,the mash water heated up really fast to 170F. Let it cool down while weighin out hops. Temop down to 152F...cool,all set. By the time I storred the grains in,broke up dough balls & all that,it went down to 149F. I tied the sleeves tight this time,unlike last time. Over the one hour mash,the temp went up to 156 even! That kettle was steamin when I took the lid off after unwraping it. So it seems any insulation must be tightly wrapped to be the most effective.
Holes or cracks in the outside skin of a cooler will make it loose insulating ability as well.