Have

@Article{Jr.2025,
  month      = {Jun},
}

and

@Article{Jr.2025,
  month         = jun,
}

The latter is better. The merge dialog should select the second column. -- See #14454 for a similar scenario (other field)

Task:

1. Create org.jabref.logic.bibtex.comparator.plausibility.MonthPlausibilityComparator
2. Wire it into org.jabref.logic.bibtex.comparator.plausibility.MonthComparatorFactory. Use FieldProperty.MONTH
3. Add test cases

Hints:

• Do test-driven development.
• Look at the failed PR Added MonthPlausibilityComparator to prefer normalized months #14629. It is 90% done, you can base on this.