That's quite a bit of a temp drop. Grain will help hold the temp somewhat but still..... What type of cooler are you using? Was the cooler warmed up prior to the start of the test? As in, prior to mashing I preheat my cooler with 130 degree water, dump it and then immediately start adding strike water and grain, so the cooler is hot/preheated prior to the mashing. I wonder if the temp loss was due to the cooler being warmed up initially with the hot water?
You might try it again with a preheated cooler?
I don't know?
Good luck