See https://forum.inaturalist.org/t/research-grade-with-only-one-id-at-that-rank/3270 I can replicate.

1. Observation is posted with 1 species level ID.
2. Another user adds a non-disagreeing ID of the parent genus and checks "No" for "Based on the evidence, can the Community Taxon still be confirmed or improved?", making the observation RG at genus.
3. Observer adds their species-level ID again, or adds an ID of a different species in the same genus.
4. Observation becomes Research Grade at the species level.