FWIW, I was able to recreate this problem by using the generate-sea:mp option in mkgmap. I normally use the generate-sea:no-mp option because this creates a nice land polygon that covers the default Garmin yellow on my GPS (my GPS ignores any attempt to change the 0x4b polygon using a typ file). With generate-sea:no-mp, I have no problems. But with generate-sea:mp and MapSource 6.16.1 , I get similar problems to those shown above, with blurry images when zooming in and out that are only resolved by refreshing the cache.

This suggests that, as extremecarver says, it must be something to do with the background polygon.