Does it also do this on the real webpage outside of the builder?
Sometimes the builder has some caching issue. Not sure if it is builder or browser cache. If it works fine outside the builder you can just ignore it. There’s this reload button inside the builder (top center) that might fix it.
Clearing browser cache inside google chrome is opening inspect tool, rechtclick on reload page, earase cache and force reload).
I’ve even tried duplicating the template, changing the post content, rebuilding the template from scratch.
There is something about post content that changes the margins and I don’t undesrtand how that can happen, considering it’s like a ‘child’ of the page template
If you choose inspect again on the margin part it should open the inspect code at the place where you click to inspect (take the top margin). It will then show all settings related to how it is made