> it gets deserialized as the zero value

It’s more complicated:

https://protobuf.dev/programming-guides/enum/

>> What happens when a program parses binary data that contains field 1 with the value 2?

>- Open enums will parse the value 2 and store it directly in the field. Accessor will report the field as being set and will return something that represents 2.

>- Closed enums will parse the value 2 and store it in the message’s unknown field set. Accessors will report the field as being unset and will return the enum’s default value.