When the KVv2 PATCH support was added, it was made to require a separate patch capability.
The PR in question - Add HTTP PATCH support to KV by ccapurso · Pull Request #12687 · hashicorp/vault · GitHub - even says so:
Despite the fact that a patch operation could be interpreted as a type of update, the update capability will continue to function as it does today.
But it doesn’t explain why!
Does anyone know an actual reason for this?