In build 1.8.2 there is an issue with rotating your IBL on the V axis in the Material Editor - it goes all stripey. 
Like this: 

This is a known issue, and on the bugfix list. In the mean time, there are three fixes: 

Workaround 1:
Use the Max native Environment under Rendering>Environment and just drag your IBL into the Map slot. Play it safe and use copy, not instance. 

Workaround 2: Go Old School. 
Old School like manual IBL setups in your scene like we used to do Way Back When(c).
Like this: 

  1. Create a sphere
  2. Play it safe and flip its normals by adding a Normal Modifier from the quad
  3. Scale to fit your scene if needed
  4.  Open the Material Editor
  5. RightClick and create a new RPR Material
  6. RightClick and create a new RPR Emissive Material, and hook it up to the RPR Material in the Surface slot
  7. Drag your IBL from the RPR Environment into the Material Editor. Play it safe, and use Copy, not Instance
  8. Hook the IBL up to the Emissvie Material's Colour slot
  9. Apply the Material to the sphere
  10. Rotate as you see fit for your image
  11.  Tweak your RPR IBL strength in Environment Lighting>Intensity
  12. Render away

Workaround 3:
 Rotate the Radeon ProRender Widget in the scene