Why can't my Mac say "defeat ISIS"?

I'm running Sierra (10.12.5) and I've been using the text-to-speech feature to read news articles. The system stumbled over the phrase "defeat ISIS" today, saying DEH-feh-CHURring EYE-sis. The system can pronounce "defeat" and "ISIS" correctly as separate words, but if I put them together, "defeat" gets the incorrect pronunciation.

While probing this oddity further I've discovered that:

Of the English speaking voices, only the Samantha voice has this flaw.
The DEH-feh-CHURring pronunciation occurs if you follow "defeat" with any word at all. Even random strings produce the error.

Is this user-fixable short of giving up on the Samantha voice?

Solution 1:

Speech Synthesis in OS X

As a user, you can affect macOS's text to speech pronunciation and approach. Everything you need to know is in Apple's Speech Synthesis in OS X.

You can provide TUNE Format information to control how the speech is generated. An Apple developer tool called Repeat After Me may also be useful in this endeavour.

Also see add pronunciation to text to speech.

Let Apple Know

However, you have encountered an interesting and serious bug in Apple's text to voice system. I recommend telling Apple's engineers directly either through the macOS feedback form or via the Bug Reporter service.

The problem can be quickly demonstrated with these two Terminal commands: