Good news: I managed to reproduce in a controlled environment the corruption of a spriteset. Bad news: because I had no idea what was the possible cause of such corruption, I just went through a random set of deletions and then a random set of recycling of the released blocks. And repeating the process from a sane initial spriteset until the "checker" tool reports something abnormal

One interesting thing, though: what's abnormal is abnormal twice: not only I shouldn't have "cross clusters" in my sprite pages (two slots on two pages actually referring to the same tiles), but I possibly shouldn't have "0" at all (it is reserved on most spriterams) and I definitely shouldn't have some odd tile number since the smallest allocation unit so far is 4 tiles.
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