Display formatter: Display label source preferred order setting not sticking

[kspurgin]

To reproduce:

• Spin up ISLE-DC instance and enable this module
• Add new field (field_testfield) with field type = Typed Labeled Text (plain)

• Set Allowed number of values to Unlimited

• Set Type Reference Settings to pull from Islandora Models vocabulary (for testing only, since that one is populated out of the box)

• Enable field_testfield in Manage form display and SAVE

• Enable field_testfield in Manage display and SAVE

• Still in Manage display, click settings gear at right for field_testfield

• Under DISPLAY LABEL SOURCE PREFERRED ORDER, drag the "Prefer the fields "Label value" when present" option to under "Ignore"

• Click "Update" then "Save" on the overall page
• Re-click the settings gear at right for field_testfield
• You should see "Prefer the fields "Label value" when present" back in the preferred order list, with no options ignored.

Effect

I'm stuck with this in the Entity view display / node.islandora_object.default config:

  field_testfield:
    weight: 2
    label: above
    settings:
      display_label_source:
        - type
        - field
        - label

I'm able to use the "Manage display" form in ECS to change the display on a field of the same field type to (in repo:

 field_identifier:
    weight: 4
    label: above
    settings:
      display_label_source:
        - type
        - field