I'm also experiencing this issue in 26.4 on two different macOS devices. I was previously using the American English Siri, Voice 4 to announce the time, but now after updating to 26.4 the voice has "reverted" back to American English Samantha. Configuring the announcement voice in Settings to use Siri again doesn't help; it will show Siri, Voice 4 as the selected announcement voice, but it still uses Samantha.