Camera only renders to textures once modified during Play (from Inspector)

I have two Cameras in my Scene. One is the default Main Camera, another is a Camera that I am using to render a batch of Meshes to a corresponding batch of RenderTextures, every frame. My code looks like:

 //executed every frame
 cam.targetTexture = renderTextureA;
 Graphics.DrawMesh (renderMeshA, matrix, renderMaterial, layer, cam);
 cam.Render();
 
 cam.targetTexture = renderTextureB;
 Graphics.DrawMesh (renderMeshB, matrix, renderMaterial, layer, cam); 
 cam.Render();
 
 cam.targetTexture = renderTextureC;
 Graphics.DrawMesh (renderMeshC, matrix, renderMaterial, layer, cam); 
 cam.Render();
 

Now, this all works perfectly, with the one caveat, that cam will only actually start rendering according to these calls once I have hit play, and modified one of its properties in the Inspector.

• It does not matter which property I modify.

• It starts working immediately upon modification of any of the properties.

• Modifying one or more of its properties via script will not cause it to start working, it has to be done manually through the inspector.

• This modification must be done during Play mode.

Has anyone ever run into anything like this? I can't find anything, either digging here or in the documentation for Camera or any of the classes it inherits from, all the way down to UnityEngine.Object.

Thanks.