That’s interesting. What kind of multimeter is the other one? Some multimeters do not perform actual RMS calculation, but just give an estimate of AC voltage based on the maximum. The other multimeter may also not capture as high a frequency as the Mooshimeter. As a test, can you compare a DC reading?
Regarding buffer mode: Android 4.4 is supported but the Android BLE interface was only released in version 4.3, 4.4 still has a lot of know bugs. Can you try tapping the “Refresh” button in the lower left corner while in buffer mode? It should not take minutes to load, expect it to take 5-10 seconds to transfer the measurement buffer.
Please let me know how it goes, best