Unless you had a very small grain bill, I suspect your mash temp may have dropped more.
IME, it is important to give some time for the mash temp to stabilize, especially if your cooler was not preheated well.
I like to mash in, stir well and let the mash sit for 3-5 minutes, then stir well again and check the temperature.
I find that if you check immediately, you will get a false high temp, as the grain has not fully absorbed the heat, and will continue to fall a few more degrees before stabilizing.
All that said, your strike seems a little high, but I would have anticipated a higher drop after adding the grain.
Just spitballing here, not positive?