Currently, only the List and Number validation rules are supported when using Workbook.ToDocument();

• https://github.com/telerik/kendo/blob/master/dpl/Telerik.Web.Spreadsheet/Models/Workbook.ToDocument.cs#L221 

It happens when the type of validation is text or custom 

for example:

If I saved :

"validation": {
                "type": "reject",
                "comparerType": "custom",
                "dataType": "custom",
                "from": "AND(LEN(A2)<3)",
                "showButton": true,
                "allowNulls": true
              }

I get back:

"validation": {
                "type": "reject",
                "comparerType": "equalTo",
                "dataType": "number",
                "from": "=AND(LEN(A2)<3)",
                "to": "\"=\"",
                "allowNulls": true
              }

It happens also when the comparer type is text - it saves it as a number

Also when I use the create validator with your UI and choose a custom formula it can't be saved because the compare type is null. As a result, I must define manually the custom validator