Problem turning off main camera (only on iOS build)

I have a map, and a gameplay level. 2 cameras, one for the map, one for the Ui and loading screen. When the user selects the level to play, the Loading screen is activated (rendered by the Ui Camera) then map gameobject and the map's Camera (which is the main camera) are turned off. Once the gameplay level is loaded, the loading screen is turned off..... Problem... When the map and Camera are disabled, the Ui camera also stops rendering for 1 frame... Why ? This doesnt happen in editor (on mac or PC, with droid or iOS settings), or on an android build, but it only happens on iOS build (multiple iOS devices tested from 4s to 6s, iPad Air) is this a unity bug ? any ideas ? Help appreciated, thanks.