I posted this on the mkgmap developers mailing list and got this reply:

Peter Hendricks wrote:

Hi,

Sorry if this has been discussed before, but I did search the list and
couldn’t find any reference.

I’m using maps from OSM, downloaded from here:
http://garmin.na1400.info/routable.php

Ever since I upgraded Mapsource to the latest I’m getting the screen
corrupted when zooming in. Details are currently discussed here:
http://forum.openstreetmap.org/viewtopic.php?pid=83694#p83694

I would have thought that the developers would be aware of this problem,
but can’t find any reference to it.

Cheers,
Peter.

I also have MapSource 6.16.1 and I don’t get this problem, so it’s not
universal. When I have been messing around with self-generated maps
in the past, I have seen this problem occur (with older versions of
Mapsource) but that was due to the bounding box of my data not being
converted to Garmin coordinates properly so the map “border” was
offset from the actual map data.

– Charlie


Is the problem with how the tiles are split, then?