According to this article https://docs.microsoft.com/en-us/stream/portal-use-deep-search when searching for a word or phrase, the search results should show results for transcriptions across all videos. However, when searching for a word or phrase, it just shows no results.
@Matt_ScuSame here. Written also a post on this: https://techcommunity.microsoft.com/t5/microsoft-stream-forum/search-in-microsoft-stream-does-not-cr... Still exists. Posted feedback to the Stream team.