3.1.6:

If custom function has a return type, create a custom property registration with the name "return" (violating the usual rules for what a registration’s name can be), a syntax of the return type, an inherit flag of "false", and no initial value. Add the registration to registrations.

3.1.11:

For each custom property registration of registrations, set its initial value to the corresponding value in argument styles, set its syntax to the universal syntax definition, and prepend a custom property to body rule with the property name and value in argument styles.

emphasis mine

This seems to erase any return type.

Should set its syntax to the universal syntax definition only apply to registrations that have corresponding argument styles, thus skipping return?

Or is set its syntax to the universal syntax definition intended to only be applied to those registrations that do not yet have any type?