Android graphical issues in pillarbox area on aspect ratio > 1.86 phones

When building our game for ultra-widescreen phones we are getting graphical issues in the pillarbox area on phones with aspect ration > 1.86. We are using 2017.4.7f1 and targetting Android API 26 with support down to 19.
Code is locking the Main Camera and UI camera viewport rect to max 1.86 aspect. Background camera is being used to create a repeating pattern within the pillarbox area. Main game shows a portion of the right-hand game view being repeated in both pillarboxes and alot of visual corruption occuring in the same areas.

We have previously tried doing this by setting the max supported aspect in the Android Manifest but this didn't work for all phones on Android 8.x. Until we do a proper ultra-widescreen UI change we need to provide a stop-gap solution, which is the pillarbox solution being attempted here.