calendarList() doesn't return all calendars (including the default one) for some calendars

For an account, the API endpoint that normally returns all of a user's calendars, graph.microsoft.com/v1.0/me/calendars, doesn't return all of the calendars, including the default calendar. What's strange, is that it does return the "Birthdays" calendar, but nothing else. In other Microsoft accounts, this GET request returns all calendars. We can't seem to figure out why for this one account the default "Calendar" calendar isn't returned.

Here is an example of a typical response for a user:

[
  {
    id: 'aaa',
    name: 'United States holidays',
    color: 'auto',
    hexColor: '',
    isDefaultCalendar: false,
    changeKey: 'changekey',
    canShare: false,
    canViewPrivateItems: true,
    canEdit: false,
    allowedOnlineMeetingProviders: [],
    defaultOnlineMeetingProvider: 'unknown',
    isTallyingResponses: false,
    isRemovable: true,
    owner: { name: 'user', address: '******@email.com' }
  },
  {
    id: 'bbb',
    name: 'Birthdays',
    color: 'auto',
    hexColor: '',
    isDefaultCalendar: false,
    changeKey: 'changekey',
    canShare: false,
    canViewPrivateItems: true,
    canEdit: false,
    allowedOnlineMeetingProviders: [],
    defaultOnlineMeetingProvider: 'unknown',
    isTallyingResponses: false,
    isRemovable: true,
    owner: { name: 'user', address: '******@email.com' }
  },
  {
    id: 'ccc',
    name: 'Calendar',
    color: 'auto',
    hexColor: '',
    isDefaultCalendar: true,
    changeKey: 'changekey',
    canShare: true,
    canViewPrivateItems: true,
    canEdit: true,
    allowedOnlineMeetingProviders: [ 'teamsForBusiness' ],
    defaultOnlineMeetingProvider: 'teamsForBusiness',
    isTallyingResponses: true,
    isRemovable: false,
    owner: { name: 'user', address: '******@email.com' }
  }
]

And here is an example of the erroneous response for the user:

[
  {
    id: 'bab',
    name: 'Birthdays',
    color: 'auto',
    hexColor: '',
    isDefaultCalendar: false,
    changeKey: 'changekey',
    canShare: false,
    canViewPrivateItems: true,
    canEdit: false,
    allowedOnlineMeetingProviders: [],
    defaultOnlineMeetingProvider: 'unknown',
    isTallyingResponses: false,
    isRemovable: true,
    owner: { name: 'user', address: '******@email.com' }
  }
]

There is no pattern to this response: we asked the user to change multiple settings in their outlook calendar and occasionally the response would include the "Calendar" calendar, but when the account is left alone, it doesn't return it. Nothing we researched could reproduce this issue locally.

Is there maybe an admin-level setting that's affecting the response?