When you buy a game, you have access to all files of the game that do not have a custom price set. This means if later on the game decides to increase its price, you will still have access to all of these files.

why do I suddenly lose access to the $10 version

When you say “$10 version”, I assume that’s a file that has a custom price of $10, instead of the price of the project itself. Files with custom price work in a Tier system, so if you spend $5, you will have access to all files of that project with a custom price of $5 or less.

Hope that helps