This is most likely due to the fact that your 720 timeline is NDF and you're going to a DF format. FCP has never supported Drop Frame in 720p and I have no answer for why or if they ever will, I've been asking for three years.
You will probably just have to shift your edit by 1 frame to ensure that it hits correctly on Field 1 on the HDCAM.
Walter Biscardi, Jr.
Biscardi Creative Media
HD and SD Production for Broadcast and Independent Productions.
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