Hello @Feanor ,
I had the same problem, with my Power App with a Home screen and details screen.
Display setting: default size, scale to fit, lock aspect ratio.
When I tried to publish in SharePoint in a 1 column section, whatever I did I had the screens truncated.
I reduced the display size to I-pad, custom, but still the screen was truncated after publishing…
I was able to fix the problem by adding a transparent rectangle with 1px border, to both home screen and details screen. After I inserted the triangle I selected ‘Reorder’ then 'send to back' for both screens.
When I published it in SharePoint the size was not truncated even after manually reducing the size occupied by the webpart.
You could try it and check if it works for you as well.