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I was missing 18. I knew that most of them were on a certain island on Roeland's map, but I couldn't get there in the actual game. But then I stumbled on the fact that the irksome pink mushrooms only throw thorns going one way!
The island with the blue dot (D3 - the one with all the fireflies) is the critical area. Access D3 from the North, and then you can go West to D2. That's where the tiles were that I'd been trying to get to all night. From D2 you can now go East and South to pick up more tiles, but you cannot go back to the East (where you came from).
Here's the whole map of that section. Note that on the island with the large ship (B4), you can also go to the East even though the pink mushrooms are there.
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Another possible location for missing tiles. D1 is not on the axe/lantern/bow pathway.
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A third possibility:
