The power supply is more than enough, but the graphics card also receives power from the motherboard’s PCIe x 16 slot, which might be the culprit if some power phase is actually bad in the motherboard.
There is a known issue which happens with several graphics card models against certain motherboards which makes the graphics card to disconnect. This affects several 3D applications and games aswel. This would happen with the editor switched to Dx12 or Dx11 no matter, so I guess this is a bad luck. The possible choices are not many, it would be interesting to know which are the graphics card and motherboard brands, and if you could borrow someone’s else motherboard and see if the issue somehow stops happening. An alternative is to check if your motherboard BIOS has received any update that could improve this, but I recommend leaving the update under capable hands because an error might leave your system unusable along with its data.
If you search on google by " D3D device being lost" you will find several reports on the engine and other applications like Substance Designer/Painter, and lots of games, some based on Unreal and others not.