Field indexes not being created

Bit of a weird one -- been debugging some performance issues with Payload and have noticed some of the indexes we defined on a field haven't been working

This doesn't seem to be happening consistently across our application, for example this field did not have an index:

{
  name: 'externalId',
  label: 'External ID',
  type: 'text',
  access: {
    read: adminOnly,
    update: () => false
  },
  index: true,
  unique: true
},

Other similar fields have had an index though, for example this slug index:

{
  name: 'slug',
  type: 'text',
  label: 'Slug',
  index: true,
  unique: true,
  admin: {
    description:
      'The unique identifier for this page. This will be used in the URL.',
    position: 'sidebar'
  },

Anyone got any insights to this?

Running

const members = payload.collections['members'].Model

await members.syncIndexes()

creates the index