Generally, when someone sells you assets to use in building and shipping a game, they’re not granting you the right to distribute those asset files individually or in an asset pack, for the obvious reason you mention, and I expect the source project of a game falls in that category.
I’ll admit I don’t have much experience with paid assets in open-source projects. But why would it be different if those assets were part of an otherwise open-source project, versus being part of a downloadable zip which is the game itself? In either case the customer can gain access to those assets. This is assuming the open-source project makes it explicit that those assets are not under the same open-source license.