This is probably not a problem most people will notice, since this is revealed only by looking at the API response for a get request that includes observation details.
If you add an observation in the web upload page and include an identification at the time of upload, it looks like the previous taxon recorded for that identification is the same as that initial identification taxon (ex. https://api.inaturalist.org/v1/observations/93816630).
If you add an observation with no identification and then later go back and add an identification, it looks like there is no previous taxon recorded for that identification (ex. https://api.inaturalist.org/v1/observations/93816629).
It seems like the behavior should be consistent, and there should not be a previous taxon recorded on the first identification, regardless of whether it was loaded with the observation or if it was added after upload.