This is a function of a change in detail in the underlying occupation codes that are harmonized into OCC1950. OCC1950 harmonizes the original codes available in the variable OCC (based on Census occupation codes, which are updated about every 10 years); CPS switched from using the 1990-based coding system to the 2000-based coding system in 2003. The coding system implemented in 2003 does not include specific subject identifiers for instructors, and instead includes only an “other teachers and instructors” category. You can see this in OCC1950 frequencies; the ‘subject not specified’ (029) code under the ‘Professors and Instructors’ category is available in all years and the frequency roughly doubles beginning in 2003 when instructors of specific subjects are no longer identified.
Depending on your specific research application and years of interest, you may be interested in the OCC1990 or OCC2010 variables, which harmonize all occupation codes to schemes derived from the 1990 and 2010 coding schemes respectively.