My best guess would be that the US purchased 50LM has had a satellite fix in North America at some time in the past whereas the other two are South American devices and haven't ever been physically used in US. Sometimes strange routing in simulation can be due to that but not by any means does it always happen. Additionally, the mere presence of the official Garmin CNNT NA HERE mapping in the nuvi 50LM (even though de-selected) may be overcoming some routing deficiency in the OpenMapChest data. Both speculative of course.
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