AC noise to electronic circuitry
If the Meanwell supply is a voltage source rather than an LED current source, then the voltages must be closely match to the load, which is not the case here.

When you drive 12V 3A limit with some C source to a 6.4V low battery the current across the switch is too high to regulate and it shuts down.  

Thus your voltage in and out must be closer matched to supply a charge current limited by RdsOn, delta V/Rs with Rs including  battery ESR( e.g. 50mx2=100mOhm), Supply (&lt;10mOhm, depending on load regulation error deltaV/Delta I) and current limit of supply after a surge 2A. 

Beware that you do not use current sensing Rs to battery, so you cannot regulate CC and CV with cutoff when you have an external load current during charging.  Warning !

Alternatively you can create a buck regulator to step down the voltage using PWM with supply suitable choke and clamp diode by storing the current I*t between pulses.