Video texture displays strange colours unless a reflection capture sphere is in the scene and HASN'T built reflections yet?!?

This is present in both 4.18 and 4.19.0 Preview 4

Using a Gear VR on a Samsung S8, when I use a media play to play a video texture, the colours appear strange, mostly blue and pink, unless there is a reflection capture present, which hasn’t had its reflections built/captured. If I hit build and update the reflection capture, the video displays incorrectly again.

Included is a video so show you what I mean. The left is with a fresh (not updated, hence the message at the top) reflection capture in the scene, the video displays correctly. If I remove the reflection capture, or update its reflection capture, the colours turn strange.

Any idea what might be causing this? Besides the player, the reflection capture and a pawn holding a sphere for the texture to map onto, there is nothing else in the scene. The video material is an unlit material with the media texture plugged into emissive and nothing else.