I may well be wrong, 'cos I’m a real newbie to UE4…just started playing with it 2 weeks ago!
I’ve found that this works for me:
- Import HDRI map
- Double-click it to open editor
- On right side, go down to Level of Detail
- Set Mip Gen Settings to No Mipmaps
- Save
Hopefully that helps and works. If it does, then I’m happy to have made at least some progress in my learning!
Good luck!