Second Camera Causes Main Camera to Go Black (iOS Only)

In updating an older iOS app I've found that having a second depth-only camera in the scene is causing the main camera to go black. This only happens on iOS, works fine in the editor.

I have two prefabs in the scene, one for the menus and one for the game itself. Each has its own cameras and lighting, and only one is active at a time. The game object has two cameras, a main camera and a depth-only camera just for a particles layer. When I deactivate the menu and activate the game, the screen goes black. I'm not doing anything to the cameras, just setting their parent object to active/inactive. If I get rid of the particle camera, it works correctly. It seems that just the presence of that second camera turns the main camera off.

This only happens when running in iOS. It works normally in the editor and worked correctly on iOS last time I updated the app. I'm running out of time to get this app updated, and I'm not sure what else to try.

Update: I found a workaround, NOT a solution, in this old thread which sounds like the same issue: https://issuetracker.unity3d.com/is...when-anti-aliasing-is-enabled?page=1#comments

Turning off MSAA for both cameras AND turning off anti-aliasing in the Quality settings fixes the problem. If this is the same issue (765563), then it was not fixed in Unity 2017.1.0.

I can live with this workaround, but it looked better in the previous version with anti-aliasing working.