[MapProxy] Mapnik Source Tile Edge Issue

[Alberto Ponte]

I have Mapproxy configured to render tiles from a mapnik source.  In the
lower zooms in this mapnik style, the tiles are rendered from GeoTIFFs.  In
these zooms, there are blank tile edges as shown in this image:
https://ibb.co/eLqOrG

When using the same mapnik style with renderd, the tiles are perfect.

Has anyone seen this before?
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