Yup.when: "`NaN` (cast from the value `NaN`)"
You should use typeError
Here is an example from my code:
amount: Yup.number() .typeError('Amount must be a number') .required("Please provide plan cost.") .min(0, "Too little") .max(5000, 'Very costly!')
Since you want the field to still validate when the value is NaN and it's not a required field, changing your otherwise to this should fix it:
otherwise: Yup.number().notRequired().nullable()
Adding the .nullable() at the end is the key.