This bug has now been fixed! You should always get consistent results when using the Lookup endpoints in the same way.
Note that changing the value of similarity_checks may change affect the answer, since this parameter affects how the query behaves as well as any temporarily-cached data.
Original answer (August 2022)
This is a known issue and we are working on improving the consistency of our results. This usually happens when our API is unsure of the result. As for inaccuracy, see this post for context.