There indeed is a default phone number within a contact, at least on Huawei EMUI phones.
However, you can not change the default when you are editing the contact.
On my P20 Pro with EMUI 9, this is how to change the default.
To change the default number, you have to open phone dialer, find the contact in the call log, touch the "i" symbol in a circle on the right, then select "Details" tab, and now you can see all the numbers assigned to that contact. Here you can long-touch the desired number, and a menu pops out with "Set default" entry.