I am having the exact same issue against a test environment we setup to test branch versioning. Our environment is 10.6.1 for what its worth. I started a support ticket with ESRI and the support analyst said he is working our issue along with another one someone else submitted to him running 10.7.1. I am hoping to hear back from them in the next few days after they do some internal testing.
For what it is worth when I published my feature service to portal I initially attempted to do so as a user ("giseditor") in the GDB that does not have writes to edit DEFAULT(as it is protected and only the schema owner, "SDE" in our case, should be able to edit DEFAULT). However, Pro through an error when I went to publish the service saying that "Version Management is enabled, but the connected geodatabase user, 'giseditor', is not the branch versioned dataset's owner, 'SDE'. Consequently I had to change that connection to the SDE user and publish my service. The outcome is a Feature Service that lets any user with access to the service edit DEFAULT. Not exactly what we were looking for. Portal users with access to that service can technically create branch versions, so that works, but there is nothing protecting DEFAULT.