Why do I have 44V AC between ground and the V(-) and V(+) of a switch mode power supply?
What you have is a power supply with floating output.
Since the output is floating, it is basically not referenced to anything in DC voltage terms - not even the mains earth - but there is stray capacitance of the switch mode transformer so for high frequencies there is an AC impedance between mains side and isolated side.
Even if mains neutral is connected to mains earth potential at some point, the switch mode supply basically has a bridge rectifier to rectify the mains AC into a capacitor. But since 120V AC mains live is has +/- 137V peaks compared to mains neutral, while the capacitor itself has always the rectified 137 VDC voltage over it, but the capacitor negative terminal will alternate between about 0V for positive peak on mains live and about -137V for negative peak on mains live. Therefore the mains side rectified DC has a 60Hz common mode voltage component compared to mains earth.
That 60Hz common mode component may pass on via transformer stray capacitance to isolated output side.
Also the isolation transformer is driven with a high frequency (eg. 20 kHz) 137Vpp square wave on the primary side, and high frequencies pass the transformer stray capacitance much better, the capacitance may couple a high frequency common mode on the output.
Therefore to combat the high frequency AC common mode on the output, there could be a so called Y capacitor between mains-rectified DC side and the isolated output side to reduce the AC ripple via transformer stray capacitance. The Y capacitor is therefore used for EMI reasons, and will shunt the stray current.
Even if the EMI capacitor is only few nanofarads, it also causes the output to wave along with the common mode of the input rectified mains voltage.
And so, basically, when you are seeing say 44 VAC from isolated supply output to mains earth, you are actually measuring the common mode voltage which the Y capacitor will couple from mains side to output side.
The multimeter you are using has extremely high input impedance (think in the order of 1 to 20 megaohms), so the small capacitor is able to push and pull the voltage over that impedance quite well. If you connect a smaller impedance (like a 1 kilo-ohm resistor or 100nF capacitor) from output to ground, the voltage measured will be much lower.
Sometimes power supplies that have earthed mains input, will also have earthed low voltage output, but if it is floating, the output will still most likely have a built-in EMI capacitor from the isolated output to earth, so the common mode voltage is already shunted inside the power supply to earth.