I didn’t really specify whether the regression to the mean was from data or tuning. It’s just happening. They are creating neutral average language instead of varied expressive.
Though there evidently was an attempt to make GPT-5 sound more neutral, but in a literal sense: the emotional content of its text was flattened out. Suddenly it wasn't nearly as sycophantic or enthusiastic, and wrote much more dry, neutral text.
I think it's more accurate to say the text is not regressing to a mean, but to an artificially constructed style defined by the training process. And the AI companies have only partial control over that style, so weird things emerge from it.