All feed has a mill date on it somewhere. It could be on the guaranteed analysis tag or on the seal strip at one end of the bag or the other.
I'm sorry you are going through this and even the most diligent among us can make a mistake or be hoodwinked. I've been doing this all my life and still (albeit rarely) nearly buy something out of date. I too have discovered feed on store shelves that was a year and even two years old. I did force a store to take back a 60# bag of poultry nutri-balanacer that was over two years old. They weren't happy about it and tried to blame the manufacturer but research on both ends showed the feed store had kept it - unsold - for over two years.
The feed store nearest me does not remove old product and leaves it in stock for the unsuspecting. I always ask for the mill date before they even bring it out of the feed shed.
Now when they see me walk in they say, "you don't want to buy it".

How unethical!
I shouldn't shop there at all but it is the closest to my house and the only thing I ever buy there is pine shavings. Even their straw is old. The inside of the bales just crumble.
Then I was happy to discover that a hardware store near me carries chicken products, including feed. It turns out their feed is over a year old also.
I must drive to another state (over an hour round trip) to get fresh feed.
I once bought a sack of barley from the aforementioned unethical feed store. I told them it was not for seeding a field but for growing fodder for feeding. They assured me that is was feed grade when they loaded it in my truck. I got it home and the tag listed all sorts of fungicides and insecticides in the seed. They took it back.
Even if your feed bag was milled this year, buying it in April means it is now old feed and should be discarded. Being from last year, one can't even call it feed. The statement that it was "new for this season" scares me and is a red flag. Feed is not seasonal. It is perishable and should be thought of as a loaf of bread, milk or other perishable food item.
The amino acids and vitamins are most degradable, especially the fat soluble vitamins. The fats added to feed can become rancid.
One problem as I see it, there is a mill date on feed but no expiration, sell by, best by or similar date. That isn't something required by the FDA as the regulatory agency for animal feed. Let the buyer beware.
I think your beef is with your feed store, not Murry McMurray, not the feed manufacturer. It is the feed store that unscrupulously sells old product to the unsuspecting.
Well over 90% of feed store employees are in a category that should never be sought out for advice on anything related to chickens or other livestock. They know nothing.
As for how to proceed, I'm sorry to say that your birds are stunted and likely will never attain their full potential. I'm not even sure that providing a fresh feed will do the job.
You need something more therapeutically supportive to kick start them.
In addition to dosing nutri-drench according to label directions, I would give the freshest starter feed you can find but also add some animal protein several times a week. I recommend some canned mackerel. Forget the cat food or dog food. Too many other things in it. They need fish and meat. Even steak, chicken or turkey.