Well, if it were a sine wave, it would be around 17V. However, if the 'transformer' is an SMPS, then goodness knows what waveform it is (so the peak could be anything from 12V upwards), and goodness knows how a simple multimeter would interpret the waveform. Such issues could, indeed, explain the OP's measurements.There might be another reason. What's the peak voltage of 12V RMS?That is obviously true. However, whilst 12V is 12V, 12V is not 16.5V which is what the OP was measuring. It's quite probable that such is the no load (minimal load) output of the ("12V") device, but I suppose it's also possible that it has become ill.
Kind Regards, John