Hi Morton,
In 5.3, the base passes are sorted first and foremost by BlendMode being Masked or not, and then by Vertex/Pixel shader hash. As such it is possible that a non standard sky could be rendered early depending on setting. However, meshes recognized as being sky materials (Setting the “Is Sky” advanced material setting) should result in sky getting dispatched after base pass opaques and may resolve the issue you are encountering.
Best regards.