Switching Regulator Voltage Dropping Unexpectedly
The Meanwell specs are Single output models:
Load regulation 10~100%  ±0.5% Vout.  100% = 3W
1% of 3.3V= 33 mV 

Your results: -300mV ~@150mA = 450mW = 15% or 3W
 - equivalent to 2 Ohms series resistance.

Possible Conclusion: faulty unit, verify input and connection losses.