The camera feed is upside down on iOS devices using Metal graphics API and with HDR enabled in Graphics settings in Unity. It looks like it is a shader problem related to platform-specific rendering differences.
UPDATE: We managed to find the solution and another related Unity bug.
First the problem is with the EasyAR shaders as initially tought is related to platform-specific rendering differences, second it is a problem with how Unity render its camera and how HDR changes that, as we mentioned in this update.
To fix this we modified all EasyAR shaders to flip the texture uv when #if UNITY_UV_STARTS_AT_TOP is true and a script that set forceIntoRenderTexture to true on the Unity camera, that is doing the same thing that enabling HDR do, except the actual HDR effect.
Here is a link with the modified shaders, just enable forceIntoRenderTexture on the Unity camera with a script and it should work.