Unable to add new Managed Metadata tag to Library items

I have SharePoint Admin permissions. There is a field I'm using called 'Publisher', which is open, yet doesn't allow us to add any new term.

The setup looks like this:

Within the Global term store:

 https://<domain>-admin.sharepoint.com/_layouts/15/online/AdminHome.aspx?modern=true#/termStoreAdminCenter

- Term group

-- Term set (Publisher) (Submission policy - Open policy: Users can add terms from a tagging application.)

--- Adidas

--- Reebok

--- Nike

Within the Site term store: 

https://<domain>.sharepoint.com/sites/<Site>/_layouts/15/SiteAdmin.aspx#/termStoreAdminCenter

there are no term groups visible.

In the Site Documents library:

Content types are enabled.

I have a Publisher column, associated with most of our content types. There's a default publisher, "Our company".

The column settings include "Allow users to type new values".

The Issue

Regardless of whether I edit in grid view, or in the Details pane, once I click enter, or exit the field, or in the tag menu, the new publisher doesn't save. Users cannot enter a new publisher term.

Debugging

I have tried toggling term store settings (closed/open).

I've tried toggling the "Require that this column contains information" (since it works for similar issue people commonly have with the 'Description' column).