Unfortunately, the reported issue is a known bug, reported in our GitHub repository on this link: https://github.com/telerik/kendo-ui-core/issues/4227. The reason for the unexpected behavior is that once we have a merged cell and change its value, all the cells in the merged range take the new value. You can check it by yourself if you unmerge a cell after setting a value in it.
I am sorry for the inconvenience, but currently, we cannot provide a workaround. I also cannot say when the bug will be fixed. Each bug is being fixed depending on its priority.
If you have any other questions related to the SpreadSheet or other components, please do not hesitate to contact us.
Regards,
Petar
Progress Telerik
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