Here's something you do not see that often, Seasonic confirmed that their FOCUS PLUS power supplies can have issues with certain graphics cards resulting in black screens under GPU stress, the issue seems related to high ripple under GPU load.

The solution is quite simple though, some modified PCIe cables with enhanced shielding make the problem go away":

-- Seasonic -- 

FOCUS PLUS and GPU potential compatibility issues

We have been notified, in limited instances, where some issues may occur when a FOCUS power supply is used in conjunction with certain high power consumption GPU or brand of VGA cards. If you experience possible compatibility issues, please check the below information and contact us for further assistance:

FOCUS PLUS & ASUS GTX970 STRIX: In very limited instances black screen may occur under heavy VGA usage. Current testing showed that certain VGA cards may emit higher than normal ripple when under heavy load and using a modified PCIe cables with enhanced shielding resolved the issue.

FOCUS PLUS & Vega 56/64: For units with serial numbers before January 2018, in rare instances, possible system shutdown may occur when GPU is under heavy load. This is possibly caused by higher than normal peak current emitted by this particular range of GPU's when under heavy load triggering the PSU’s internal protection safety.

Seasonic Issues Warning: FOCUS PLUS PSU has GPU Compatibility Issues