Quick question about ec/ppm

Hi thanks everyone for all the great information you continue to share!!

I am confused about mixing nutrients and the ppm/ec readings I get in return.

am I supposed to go off the fresh mix ec reading or the mix that sits for a while reading?

for example when I freshly mix my nutes and test it the ec/ppm will be for example; 242 ppm - .48 ec.

So I will then put my gallon container in front of my heater over night to make sure the water is warm enough before watering. The next day when I test the gallon after it sat overnight my readings are 1.5 ec and 787ppm

does anyone know why the ec/ppm changes so drastically? Is it because the water is going from cold (60 degrees) when I first test it to warm (75 degree). Which reading should I trust and go off of? My plants look great other than some nutrient burn. so that’s why I was trying to lower my ec.
The ph remains the same from both tests.. not sure why my ppm/ec changes.
Thank you!!!

Are you using dry nutes?
The only reason I could think of would be some portion of the concentrated didn't dissolve completely until a few hours later.