DataGrid Known Issues - Validation Limitations
There are some limitations with cell validation when the ValidationStep is set to UpdatedValue or CommittedValue. If you are using cell validation (either by itself or in conjuction with row validation) and are experiencing issues, we reommend that you use
RawProposed or ConvertedProposed instead. Cell validation using IDataErrorInfo is broken in V1 since unfortunately IDataErrorInfo only works at the UpdatedValue or CommittedValue step, and there is no work-around available at this time.
In general, our recommended strategy around validation is that for RowValidationRules, you should use ValidationStep.UpdateValue and ValidationStep.CommitValue, and for CellValidationRules you should use ValidationStep.RawProposedValue and ValidationStep.ConvertedValue.