The same happened/happens to me (another from 4/2016 3 days ago) so I E-mailed support weeks ago and was satisfied with their answer. I am not sure if it’s proper to copy and paste the message they sent to me but they basically said:
The vitamin blend starts to lose its potency after a year from it’s manufacturing date and by how much is not clear.
It seems that they produced too much of the powdered product and are trying to get rid of their stocks in the orders that they receive. I am unsure just how long it will continue being this way.
I also asked if freezing it would harm Soylent powder. They said no. It’s still safe to consume and you’ll still get plenty of nutrients I am betting. It’s not that the powder goes bad and becomes unsafe to consume. It’s that the vitamin blend loses potency the same way old tylenol in the cubbard may still be good but have less of a kick to it.