I noticed this too!
I have a daily automation that sends me an audio message using the gpt-4o-mini-tts model along with a fixed set of instructions for emotion and tone.
The audio I received on Tuesday (1/13/25) sounded great and matched the expected tone. However, the one from Wednesday (1/14/25) was awful!! Completely monotone, no expression at all. Just flat and boring.
I re-ran the automation manually, in case it was a one-off execution issue, but that didn’t help! Then on Thursday (1/16/25), I got the same strange result again.
I tried the recommendation mentioned in this thread and switched to the gpt-4o-mini-tts-2025-03-20 model. After a few manual runs, the audio now seems to have the correct tone and emotion based on the instructions, and the results are consistent with what I was getting before.