All-day events showing over two days

I have iCloud synced across 2 devices and have recently been using the web-based version to update calendar events. However, when I tick the box to make an event 'all day', it displays across two days - but not consistently across both devices / iCloud web. (My iPhone 8 shows all-day events correctly across just 1 day.) Apparently this has been an ongoing issue as there are questions about this dating back 16 years (2008). Please can you advise how I fix this?