P905 same issue here, caused by the book cover ofcoz! Makes me mad.
Are you guys sure that it's the magnet sensor?
Well, the issue seems to be related to the horizontal position of the tablet (it starts going crazy only when lying flat on the surface - BUT also when I hold it "in the air" in a same, "lying" position), which is determined by a sensor called accelerometer, which is responsible for determining the rotation of the device relative to the source of the gravity (ie. the core of the Earth) ;] That is why I suppose that the issue is caused by accelerometer, and not by magnetic sensor!
afaik magnet sensor is included to provide compass-related functionality (ie. determine the rotation of the device relative to the position of the magnetic pole of the Earth) and may help to determine the directions of the world, but not the rotation of the device - and the issue seems to happen irrespectively to the direction in which the tablet is heading (north, south, etc.) ;] I'm not an Einstein so I may be wrong, maybe someone has enough knowledge to confirm that.
Another thing: the magnet built in into the cover is causing it when the cover is fully opened and folded to the back of the device. But it is not causing it alone. Well, I am not a professional physician, but I think that the magnetic field created by the cover when it's turned back - and in result, its influence on device's hardware - has to be quite similiar neverthless of the device rotation, as the magnetic field does not depend on gravity or something. And here, the magnet turns off the screen ONLY when the device is "rotated" in a particular way, ie. lying flat...
And how the hell does it interfere with accelerometer? Afaik it's rather strictly mechanical part, not an electric one, at most the electric components are responsible for converting the readings to a digital form and nothing more. Also, the rear cover is quite thick, and it also shall weaken the magnetic field before it reaches the device.
In summary: I can see no relation between both (and both vital...) factors causing the issue. So, I also don't believe that the book cover is faulty. Of course, I may be simply wrong but...
Well, I wonder if the issue is not really... an unfinished feature which has not been fully implemented (and also not fully removed from retail firmware) ;] Something like: "the tablet will be on when kept in hand; when you finish your work, put the tablet flat on the table and it will turn off automatically", Smart Keep or something, hmmm? Maybe the issue can be solved with a software fix, ie. some configuration of sensors within /system/etc files or by changing some values in the settings database...?