[MapProxy] Reprojecting WebMercator

[julien.kazemifard at visioterra.fr]

Sorry, my Mac changed the gif to include it in the body of the mail, 
here is the real animated gif as attachment.

Does someone have an idea on how to do such reprojection without this 
problem ?
I noticed this happens for every reprojection I try to do from EPSG:3857 
to EPSG:4326,  for example OSM and Bing,
But only for the first levels...

Is this a bug I should report on MapProxy's Github ?
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