This item has been identified as an outstanding known issue. A workaround is provided until the issue is officially fixed.
If a cell containing a value of "NaN" (not a number) is edited to contain an actual number; then using the 'Undo' feature on this cell will change value to "0" (zero) instead of "NaN".
The Undo dialog also is incorrect in this instance - it will state the value has been undone back to "NaN".
To workaround the issue, and return the cell value back to "NaN", use the model history to restore the model back to before the change.
If this workaround is not suitable in your use case, please contact Support at firstname.lastname@example.org.
Internal Reference: CORESRV-4007