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I've just pushed a fix for this in version 1.0.1. Sorry for not catching this earlier! 

For the technical details: I used the `tag` field of a sprite set to its tile’s value to check for collisions. (If two tiles have the same tag, they should merge.) But little did I know that this value is limited to 255 (as an 8-bit integer). So any tile above that would get assigned a value modulo 256. I think in your case it might have been a 1024 block merged with a 256 block, which both would have had a 0 value tag. I've fixed this by using the log2 value of the tile instead of its full value, increasing the possible value up until 2^256 which… should be fine.