...my gut feeling is that it's an issue between Mapsource and the maps, maybe just some tiles...
Devices are updated unlike Mapsource.
Did you simulate it?
I have figured out how to save separate files of data to keep some of my information separate.
C:\Users\<Username>\AppData\Roaming\Garmin\BaseCamp\Database\<Version>
I know, I have 60CSx with firmware 4.0. Would be interesting to make a test with it. But this is not a device sold with City Navigator and Lifetime.
As for nuvis, I think all have been updated recently except the oldest nuvi 3xx/6xx. Maybe someone can test problematic routes on 3xx or 6xx?
I hardly suggest not to update old Nuvis like 350 to CNEU 2013.40, risk to brick them routing in some zones is too high