Dear @PatrikS,
Your initial post compared the configuration dialog of the JSON to Table node with the Ungroup node. Since they do two very different things, their configuration dialogs are supposed to look different. To make sure, I just checked on a nightly version, and the JSON to Table node has not been migrated to the modern UI.
In order to help you figure out the issue, please provide some (valid!) JSON examples that are returned by each of the two GET Request nodes. It would be ideal if you could also provide a minimal, reproducible workflow example with the nodes of your two branches. Without the examples at least, there is basically nothing that we can do to help.
Best regards,
Stefan