Thanks for these! Indeed I understand your point now. I'll add a little note on the post later today.

Another interesting bit is that It wasn't so easy to get an image showing the issue a clear as we had it. My hunch is that moiré patterns might have a role in this, as most images where the the issue showed, had some kind of grid/repetition. I suppose some images might be more affected by the IDCT and other more affected by the scaling algorithm, making it more or less visible. It's a mix like you said!