The rules for min() and max() do not make it clear what the result type is, in the situation where some or all of the items in the input sequence are of a derived numeric type. For example, if all the items in the sequence are of type my:hatsize which is derived from xs:integer, is the result of type my:hatsize or of type xs:integer?

The relevant statement is "For numeric values, the numeric promotion rules defined in 6.2 Operators on Numeric Values are used to promote all values to a single common type.". But if you follow the references on numeric promotion, you get to XPath Appendix B.2, which is written on the assumption that there is a required type, which is not the case here. So this rule doesn't tell us what the "single common type" should be.

Note that if the items in the sequence all belong to a type derived from string or from date, the result of the function will be one of the items in the sequence with its original type. One would therefore expect the same behaviour for a homogenous sequence of derived numeric values.