Yes, that's right.
When you signed up for the subscription, you had access to more models for download. Now there’s a higher tier, and most of the good models have rightfully moved to it.
This is fairer to the designers (if I may say so), but less fair to you and everyone who has already purchased the subscription (I don't recall any mention of the Premium plan in CGTrader's spring newsletter).
More specifically, almost all models on the site are now available through the Premium plan, unless the designer has withdrawn from the subscription program (many designers missed that email and haven’t yet noticed that CGTrader is distributing their models via subscription).
If a designer has withdrawn from the program after you’ve added a model to “My Purchases,” you will no longer be able to download it. I’m not sure if this is a bug. So, I recommend downloading models immediately or within a few days after adding them to “My Purchases.”
Happy shopping.