You know what happens when you get chipboard wet? It swells, breaks down and loses its integrity near the surface. The powdered adhesive is wet so as you suspected, that's why you overboard chipboard. Also, grout and adhesive is porous and so will let moisture pass through it if it gets wet enough - if this happens with WBP ply, you're ok. Not so with chipboard.
Basically imagine tiling on chipboard to be similar to tiling onto weetabix.
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