Remixing cached images on-site costs no buzz... or does it?

Those using the on-site generator have probably noticed when you remix an old gen of yours it doesn't charge you any buzz, assuming it's an exact remix (same setup, including seed).
That's because the site caches all gens, and there's no charge for retrieving an already-cached image instead of using site resources to gen anew.

Two questions:

it follows that this isn't limited to your own gens; any images on-site will cost no buzz if replicated. But when I tried this out now, I see I'm usually charged for remixing others's gens.

Anyone know official policy? and if it's a bug, anyone know a way around it?

Other question: recently (AFAICT since the recent major front-end update a month ago) I'm usually being charged for remixing my own images, even when fully replicating cached stuff in my queue. Support says it's a bug, their solution isn't helping. Anyone know a solution?

And even without a fix, I'd love to hear if others here have been encountering the same issue.