I didn’t use the large weather widget long enough to have noticed that issue. But I see you have the same bug that has stopped me using the large widget. That being the upcoming days forecast showing a moon or partly moony day (as per Thursday to Saturday in your screenshot), instead of sunny or partly sunny. (Within the weather app, the icons are correct.)
When I first add the widget, the location defaults to London (being in the UK) and the upcoming days forecast correctly shows sunny or partly sunny icons. When I change the location of the widget to either my actual city or ‘use my current location‘, then what should be sunny or partly sunny displays on the widget as a moon or partly moony day.
This is on both my iPad and iPhone, both on 14.0.1.