Hello Ryan !
Thank you for posting on Microsoft Learn Q&A.
It didn’t translate because auto-detect failed. What I understood is that you omitted from=..., so Translator tried to auto-detect the source language and the response shows detectedLanguage.language: "en" with score: 0.0. A score near 0 means detection failed and it had no confidence.
FYI, some translator deployments fall back to a suggested source when detection fails and since the service then considered the source to be English and your target was also English, it simply echoed your original text back unchanged.
You need to force the source language which means you need to explicitly add from=zh-Hans or from=zh-Hant if traditional and keep textType=plain.