So I know that if you mash too thin you lower the efficency of the enzymes (or something along those lines) and mess with the conversion of starches to fermentable sugars. So I get why you wouldn't mash 9 pounds of grain with, say, 10 gallons of water.
However, yesterday I was making a mild with under 9 pounds of grain and I mashed with 3.5 gallons of water. When I went to collect my first runnings it was stuck, and I thought the small amount of water in my 70 qt cooler might have something to do with it (first time I've ever had it stuck). So, I added the water I had intended to sparge with, stirred and waited 10 minutes and collected my 6.5 gallons to boil with no problem. I took a ph reading at room temp from a little bit of the runnings collected at the beginning and a little collected at the end and according to my strip they both tested between 5.0 and 5.4.
Will this cause any issues? And if not, what is reason behind collecting your first runnings, adding the sparge water and then waiting before collecting the rest of your wort?