When having a List with a metadata column, you can define a default value. This default value is showing up in the new Form until you edit any other column in the new Form. By inserting any value, in the Form, the default value on the metadata column is removed. -> default values are useless in this case!
-> Bug that should be fixed.
-> Expected behaviour, default values with metadata fields should work the same as example text fields.

It was suggested by technet support to add this entry:
https://social.technet.microsoft.com/Forums/en-US/onlineservicessharepoint/thread/f5ace8da-d0af-4cd6-8f60-701d6c6a4333/