Your lens provides a code to the camera (a pair of numbers) that is recorded in the makernotes section of the EXIF. This is done rather than writing a model name in the LensModel tag of the standard EXIF. This pattern is common across brands and is not just a Pentax thing. Software that supports Pentax makernotes contains a table that maps the codes to lens model names. When a lens is newer than the software, the lookup table lacks the code and the software reports the lens model information as missing.
A software update generally solves the issue.
Addendum: Windows Explorer will generally not display the lens model unless it has been written to the LenModel tag by software as part of post processing.* I believe the same is true of the Mac Finder as well, though I may be wrong on that count (not a Mac user

).
Steve
* ...and even then, Explorer may fail to make that information available.