I am using the TTS and STT services of Google Cloud API together. I am inquiring because an error has occurred only in the TTS function since last week without code modification or redistribution. When I checked the data from the backend log, I found no abnormality and the Google Cloud server is sending 400 errors. There is no problem with STT, but only TTS is experiencing problems. Please check.
You may raise an issue in this Public Issue Tracker. Engineers that is working closely to the said products will check your case.