This is standard on PS+, if you get a "free" game via PS+ (all games received for "free" are tied to your subscription, PS+ discounted ones aren't) and you buy DLC for it, the DLC becomes unusable if you end your PS+ subscription. Sony tells you that when you hit the download button for one of those "free" games. You can buy the base game later if you don't want to continue paying for the PS+ subscription, and then your DLC will be usable on that game. I am a PS+ subscriber, I avoid to buy DLC for the "free" games for this reason, but everything is perfectly clear on the store when you buy.