Hi Graham,
Please ensure your Microsoft 365 apps are fully updated, sometimes updating Office fixes issues. To check version information, open any Office app, such as Excel or Word, click File>Account, check for the info mentioned under Product Information.
If the version is not updated, please install updates and check if the issue remains. To know how to install updates, follow the below link:
Try opening Excel in safe mode, insert some content in multiple cells and then select the cells and press the Delete key to confirm if the content is cleared or not.
To know how to run the Excel app in safe mode, Please follow the below link:
Thanks,
Suyash Thomson