I've worked out that the problem stems from my image being non-square. When gimp created mipmaps it made levels/layers from 256x128 through 2x1 but the final layer was 1x1. I looked at the source and found that when exporting existing layers as mipmaps it validates the layers by looking for a chain with each successive layer being half the size of the previous, using a bitshift for a fast divide by 2, so when looking for the next layer after 2x1 it would try to find 1x0 and fail.
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