Do you have the psu user manual?
If yes, does it describe how to use the switch that lets you select the single versus multi-rail option?

The following is from the JonnyGuru Silverstone Zeus ZM1350 psu review:



And the other side. I see a label I should probably have Mr. Fuji zoom in on.



The label turns out to be one that tells you how the voltage adjustments work, and how to switch into single or multi-rail mode.
It also warns you not to flip that switch when the unit is operating. Good advice.



The front panel with all the modular connectors. Blue is for the PCI-E cables, black is for everything else.
I like that the OCP switch is enclosed - less chance of flipping it accidentally.

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In the last photo, the 12 volt rail switch is on the left side.
I'm betting that your psu has a similar single versus multi-rail switch setup.
note: It appears that your 1400 watt psu does not have the Voltage Adjustment Pots described in the label.