I found the searchDiscover method is broken. Plex now requires a new parameter called searchProviders to be provided in the query. This parameter is a comma separated string with the following values:

• discovery: This is the default behavior matching the previous functionality
• PLEXAVOD: Plex ad-supported video service
• PLEXTVOD: Plex transactional video service. I've also seen this referred to as Plex Rentals.

I've updated the searchDiscover method to default to just "discovery", with the option for users to include the other providers if they wish.