Bloom in the viewport compositor is actually similar to how it works in Eevee. It’s also a temporary solution according to the devs, so it might change in the future.
Bloom effect is also affected by the camera’s passepartout setting.
Because in the compositor you’re not using any render passes, like the alpha or the noisy image output of your render layers node.
As of now viewport compositor is still a WIP. It’s good for simpler stuff like color correction, But theres still a lot of things that needs to be worked out.