[throwdbaaway]

In your example, the WHERE clause of the query and the partial index didn't match logically, i.e. the query may return rows that are not indexed. There's nothing that postgres can do, and I wouldn't classify the behavior as peculiar.

On the other hand, with sqlite, the WHERE clause of the query and the partial index must match *literally*. So let's say you have a partial index with WHERE severity_id != 0, and a query with WHERE severity_id = 1. All the rows with severity_id = 1 are already indexed, but the engine is still not able to make use of the partial index. This one bit us hard.