Thanks for providing us some details.
Well, this is MS Excel’s behavior which overrides the data validation applied (to the cell) when one copies a value from other cell and paste it onto it (in the worksheet), you may confirm this behavior manually in Ms Excel. Well, if you need to evaluate programmatically if a value (provided to the cell) satisfies the data validation rule or not, you may do it via Aspose.Cells APIs at runtime. In short if temporarily inserted value to the cell does not satisfies the data validation rule, you should not insert/ retain that value. See the document for your reference: