I suspect you are on the right track looking at the profile. It would be interesting to know what profile it uses, when you specify
AUTO, on the Linux devices with problems.
I am not aware of constraints on back buffer size. But there are limits on texture size depending on profile. I would not be surprised if the back buffer is constrained by the same limits as textures.
For one with a
RenderTexture you can treat a texture as a draw buffer, which implies that on some level it treats textures and render buffers the same way.
And normally you want textures to be larger than the back buffer, to allow for large texture atlases. Only part may be used at once, or elements from it might be drawn overlapping or scaled due to 3D mapping, so you need more texture space than pixels on screen generally.
I.e. no-one might have considered that you would have a back buffer bigger than the largest texture. So it might be the limit is the same as for textures, which with certain profiles is 2048.
Worth raising as a feature request perhaps.