It seems that when you use the admin plugin and use the expert mode, if you save a taxonomy it gets saved incorrectly.

An example on a pure vanilla install of the latest grav and admin plugin. Make any page, click on expert mode, and then add the following:

taxonomy:
category: blog

Now, once the page loads back up after you hit "Save" you'll see the following is what gets saved:

taxonomy:
    category:
        - b