Bug Render textures not working in WebGL?

I'm using a render texture generated from a second camera for a minimap. It works well in editor but I get a black texture on the build. The web console shows no errors and everything is otherwise fine. This is a URP project on 2021.3.8

Is there a secret method of working with render textures in WebGL? Currently the texture is an asset, not created at runtime. Is this not supported in WebGL? Is it an issue with the texture format?

 

Is that particular format supported?

You can check with this method:
https://docs.unity3d.com/ScriptReference/SystemInfo.SupportsRenderTextureFormat.html

Needs to be run in a build.