Hey Slotdev,
I've just had a similar issue while trying to create an Icon file from one in the resource/application.
My resulting "Icon" is 22 bytes smaller, I noticed though that it's the first 22 bytes which are missing.
Perhaps this explains why you are not getting 4D42 in the first two bytes, because the first 14 bytes are actually being missed, as apposed to the last 14.
I haven't found a solution yet, but will keep you updated if I find anything.
I realize this post is roughly a month old, so if you have found a way to fix the issue, please let me know.
Regards,
RC