alpha buffer postprocess + iOS

We recently created a custom postprocess effect that uses the alpha buffer. It works on PC but we've found that on iOS it fails to work. It looks like the reason is the render buffer format on iOS is 32 bit but no alpha for us. Is there any way to change this? On Android there appears to be a "Use 32-bit Display Buffer" setting and in the docs it mentions it can be used for alpha on postprocess effects, but iOS doesn't appear to have that.

We're on Unity 2019.4.31f1 + built-in pipeline / forward renderer.

 

For anybody playing at home, got this working by turning off HDR on the camera and making sure to include the postprocess shader in Always Included Shaders in the project graphics settings (which I shoulda done to begin with).