How have you measured the voltage?
Can you measure what the old one was supplying?
No - the old transformer was totally dead, says 12v on it.
New one I've not measured, it says 11.5v on it. Everyone seems to sell 11.4-11.6v ones now; 12v seems hard to find!?
Neither this nor the old one sound like transformers at all, but switch mode power supplies that operate at 10s of kHz with a non sinusoidal waveform. You are not going to be able to measure the RMS output voltage with the average multimeter.
If you really want 12volt output you are going to have to find a real wound transformer.