SQL Server - materialised (indexed) view not working as expected
Like @sp_BlitzErik noted in the comments, you need to use the WITH (NOEXPAND) hint, like so:

SELECT something, something
FROM the.indexed_view WITH (NOEXPAND);


When SQL Server reads your query and replaces the name of the view with the definition of the view, that's what's known as "expanding" the view, so with NOEXPAND, you specifically tell SQL Server to not expand the view, but use the index that's there.

On the flip side, if there's no index (like if someone does ALTER VIEW!) the query will fail if you use WITH NOEXPAND.