Some newer dip tubes have a closed end... If there is something in the end of the tube clogging it, it can cause cold water to spray in a different direction, dropping the temperature of the tank..
Its rare to see a tube fall off, but its possible.. Only way to be sure is to take out the cold fitting, and check the tube by removing it with a set of needlenose.
I`d check for a cross connect first.. Turn off the cold valve feeding the heater, and make sure the hot pressure stops flowing. If it doesnt, you have cold water getting into the hot system somewhere, which will also drop the temp..
if it aint broke, tweak it!!
currently on FiOS (kick aZZ!)