That may be a dealer coding issue or something related to the age of the instrument cluster.

In my old 1998 740iL, the OBC selection disappeared every time I restarted the car, just like you are describing. However, at some point in the life of the car, it became persistent. I can not remember for sure what the change was, but it was either a navigation computer OS update done by the dealer, coupled with some coding changes, or it was when I got the instrument cluster replaced. I think that it was the new instrument cluster. So, if you have an early X5, it may be the instrument cluster.

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2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input