Composite field values remain displayed as mixed when their showMixedValue is set back to false

Reproduction steps:
1. Open the “MixedValues“ project
2. Window > TestWindow
3. Click “Set Mixed Value“
4. Click “Set Numeric Value“ and observe the “Vector Field” subfields

Expected result: X and Y are displayed as numbers
Actual result: X and Y are displayed as mixed values (i.e. “—”)

Reproduced in: 2021.3.23f1, 2022.2.16f1, 2023.1.0b13, 2023.2.0a12
Could not test in: 2020.3.47f1 (“'FloatField' does not contain a definition for 'showMixedValue'“ error)

Reproduced using: macOS 13.2.1 (Intel), Linux 6.2.10 (by the reporter)

Notes:
1. Setting showMixedValue on the composite field itself rather than on its subfields doesn't work but it’s because IMixedValueSupport shouldn't be implemented at the BaseField level which is considered technical debt
2. The issue is that composite fields (e.g. Vector2Field) don't know that their subfields were modified (e.g. by setting their showMixedValue)
3. A solution could be to call UpdateDisplay() on the Vector2Field to force an update but that method is private