Sorry I don’t understand the issue you are having. You should expect the temperature to decrease since you have an endothermic reaction. If it is decreasing to fast compared with some reference data you have, then you need to double check the heat of formation for each species or the heat of reaction if you manually specify it. Maybe your reaction rate is too large so you should also verify it.
Try setting the heat of reaction to zero as I suggested in another post to see if this makes a difference: