I would regard this as a bug, personally.
If you've bought something here - even for nothing - you don't expect it to disappear like this.
Viewing post in dowload for free again?
Downloading something for free does not count as “buying” something. If you do buy (by spending money) something while the minimum price is at 0 then you will own it, even if a price is added at a later time that is above what you originally paid.
Additionally, it’s possible for developer to host a special sale where the price is 100% off but someone can claim the page to their library permanently.
Otherwise, you don’t own the game if you downloaded it once for free. Often developers change what they have available on a project page quite significantly as they develop the game, and we encourage them to add a price if that’s what they think is appropriate for their work, and run any necessary promotions or give out keys if needed.