That would be the case, if the "makernotes" were merely a binary dump of lens-supplied data. But the "makernotes" are Pentax-specific, and created by actively processing the lens-supplied data and various other data created by the camera itself.
Here's a table of what appears in the standard field and in the maker notes, for various increasing focal lenght, first the standard EXIF field value and then the maker notes value:
Code:
Focal Length : 0.0 mm
Focal Length : 8.1 mm
Focal Length : 0.0 mm
Focal Length : 9.4 mm
Focal Length : 10.0 mm
Focal Length : 10.0 mm
Focal Length : 12.0 mm
Focal Length : 11.9 mm
Focal Length : 16.0 mm
Focal Length : 16.2 mm
(created by using "exiftool -FocalLength -copy2:FocalLength *.DNG", file names omitted for sake of brevity)
Since the firmware's inability to insert the focal length into EXIF data depends on the actual focal length used, this is definitively a firmware bug, and not a lens-camera communications issue.
Sorry for being so stubborn on that issue, but I'm a senior software engineer and part of my job is to locate and analyze errors in various types of communicating processes and machines, thus I'm quite sure about what I insist upon here.