I don't know if others have noticed this or not but the MB washer fluid does have a distinctive smell; sweet, sweet alcohol, very strong, that no other brand even comes close to. You can almost catch a buzz when you spray your windshield
This tells me that the MB fluid has a much higher concentration of alcohol (used to prevent freezing) than others do. I'd say if the stuff you bought froze at 15F then it was not mixed correctly at the factory, they probably forgot to add the alcohol or just didn't add enough...I've never had any problems with any brands I've ever used but it is possible they can make a batch wrong. I once went to a plant that was making the stuff...it is a VERY low-tech process and could easily be done wrong. They basically blend concentrated generic window cleaner that comes in 55 gallon drums with alcohol and bottle it.
Anyway, the advice to use MB brand is a good one, or just try some other brand as I am sure you won't have lightning strike twice.