Headphones

I don't know if it produces more power (and I doubt you hit its power limits with with IEMs, if you can still hear anything), but it definitely does not produce more voltage - I tested this myself a few days ago. And why would it? It's the same device, the DAC isn't in the phone. More likely Apple changed the volume taper in the UI or something.

Did you by any chance replace an EU model with one from another region? The EU ones are known to be capped at half the output voltage as elsewhere due to nanny legislation.