It definitely seems odd that the number of physicians/surgeons is nearly halved between 2019 and 2020. The Census Bureau provides crosswalks for changes to occupational coding schemes, including the transition from the 2010 to the 2018 Census occupation codes, on this page. I also can’t identify some other category that is hiding in the crosswalk. Regarding the bizarre breakdown, the broad Census occupation category “3090 Other physicians” does include multiple standard occupation classification (SOC) codes that do provide more detail (e.g., “Anesthesiologists,” “Cardiologists”). However, this doesn’t offer much in the way of consolation if you are using Census occupation codes instead of SOC codes.
At this point, I can confirm that the drop isn’t a result of an IPUMS coding error, and that the issue is present in the original BLS version of the data. Our IPUMS CPS team linked the December 2019 (uses the 2010 Census occupation codes) and January 2020 (uses the 2018 Census occupation codes) basic monthly data to see if that offered any hints, and found that less than half of linked persons with an OCC code of 3060 in December 2019 had occupation codes in the January 2020 that you might expect based on the crosswalk. Many of the non-matching cases have physician assistant or medical technician occupation codes in January of 2020. At this point, the best suggestion I have is that you contact BLS, as they are the original data provider. The IPUMS CPS team is also looking into this, and I will certainly post an update if we receive more information.