A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
According to my research, what you’re seeing matches the current behavior of the modern Microsoft Lists calendar experience.
If the calendar view is already configured with both Start Date and End Date columns, an event running from 15 Sep to 17 Sep may still appear as individual entries on each day rather than as a single continuous bar like the classic SharePoint calendar. The modern Lists calendar does not provide a setting to switch back to the classic multi-day spanning display.
Create a calendar view from a list | Microsoft Support
A quick check would be:
- Edit the calendar view.
- Confirm Start Date is mapped to the event start column.
- Confirm End Date is mapped to the event end column.
If those settings are already in place, the display you're seeing would be related to the design of the modern calendar experience rather than a configuration issue. The modern Microsoft Lists calendar does not offer an option to replicate the classic SharePoint 2013 multi-day event appearance.
If having a single continuous multi-day event bar similar to the classic SharePoint calendar is important for your scenario, you may also wish to share your suggestion through the Microsoft Feedback portal so it can be reviewed by the product team.
https://feedbackportal.microsoft.com/
I hope this information helpful and if you have any further questions, please feel free to ask via comment section.