Difference in Eevee between viewport and render

3D Viewport:


Rendered Image:


I suspect this is an SSR overscan issue? But where can I change this setting?

Not sure, wether this will actually fix your issue, but the overscan option is located in the Film panel within the Render Properties.
There is a soft cap at 10% but it can manually be set to up to 50%.

Thanks for this! I bet that will be useful to me in the future.

But I now suspect that the above comparison I posted was more likely a samples issue. Cranking up the viewport samples makes the reflection also appear before the render… but it’s still way more visible in the final render, but at least I now see it…