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I've not replaced a rear bearing in an E90, but I do know from reading the procedure that it needs to be installed in the correct orientation, meaning the "magnetic" ring must face inside. I have replaced a front hub (bearing) and it does use a traditional toothed gear/hall effect setup. Not being familiar with the specific design of the rear bearing, it could be that BMW just uses the actual ball bearings (evenly spaced in the bearing cage) as the "teeth" of a traditional wheel speed sensor setup. I would expect that the "magnetic" side of the bearing that needs to face the transducer is just that the seal is magnetically transparent so the transducer can detect the ball bearings. Now if the bearing is starting to fail, where the ball bearing cage has broken and the balls are no longer evenly spaced, that would cause a malfunction of the wheel speed sensor.
If you've got CPO, then I'm not sure why you took it some other place first.
Good luck with it.
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