I ran into the same problem and found a solution. First, I did the following: I placed the custom ringtone in the "Music" folder, then added it via "Add ringtone" in the settings and selected it (all of this in a separate user profile). The ringtone didn't apply. Next, I did the same thing in the Owner profile, but that didn't help either.
Then I did the following: I went to Settings > Apps > See all apps > searched for "Phone" (black-and-white icon) > Permissions > Music and audio > and set it to "Allow." After that, the problem went away. I later revoked that permission, and the ringtone still worked.