Personally I think this needs further investigation - we have discussed this on another site as you know
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I gave it some further investigation. I deleted DrawPriority settings from my local tile, sent it to the nuvi, and, much to my dismay

, the tile overlaid the DEM basemap, just fine. Of course, in the meantime, I rebuilt the whole set to DrawPriority 31, and in so doing found a number of multiple dictionaries. Maybe those were causing the problem.