Corrosion etc.
Corrosion anywhere on the bike is dissapointing. Unfortunatelly our beloved machines age and so do we.
I got my first experience with rust on a weld of a main frame, when the bike was few weeks old. It was at John's Clarks, Dundee and the guys changed the whole frame as warranty job. This may sound like good news, but as the new frame was not numbered, they had to taper the numbers which broke the paint coating of the frame and it is gradually rusting there. So after all the hustle, I did not get rid of rust anyway. I just god it on different place.
My engine's paint serves well, but over time there is some paint flicking off the oil sump. I have done repair myself while the exposed was still small.
In ideal world, the enamel coating should give good protection of the aluminium engine body under and in most cases it does. Unfortunatelly as there as many scenarios as there are riders, it does not work always.
Reports on this site clearly show, that there are/were some quality issues on side of BMW. While ACF50 and similars can help prevent corrosion, there are other parameters which can't be affected.
As in case of every bussiness, sorting corrosion issues under warranty can be tricky, especially when the bike is older and it is difficult to distinguish between natural lever of wear and tear and corrosion due to the poor finish and material failure. As general rule, they will try to do as little as possible.
While it would be nice if they accept the liability in all cases, I would not say they are unreasonable.
While rusted engine is a personal tragedy, for them, it is just one bike that was not produced up to the good standards.