I had the same problem and started to look for a solution when I found this thread and was waiting for a replay.
During that time, I looked further into BC's HELP. They stated that they were 2 ways to transfer BC data to the Garmin, I have a 2455 by the way.
#1 is the EXPORT method that kept giving me gave me the broken route problem, as described.
#2 was the "Edit - Send to" Option. I routed a 500 mile back-road route to Boone, NC on BC and transferred to the Nuvi. The route, on the Nuvi, appears continues and correct.
I will be leaving tomorrow on my trip and will let you know in a couple of days.:)