Why do art historians disagree about the Assisi frescoes attributed to Giotto?

The debate surrounding the Assisi frescoes, often called the "Assisi problem," centers on whether Giotto actually painted the cycle of 28 scenes depicting the life of St. Francis of Assisi in the upper church of San Francesco.1

While a long-standing tradition—supported by early writers like Riccobaldo Ferrarese and later Giorgio Vasari—attributes these works to Giotto, modern scholars are divided for several reasons:

• Stylistic Discrepancies: Many art historians argue that the style of the St. Francis cycle is irreconcilable with the Arena Chapel frescoes, which are universally accepted as Giotto’s work. Critics suggest the Assisi cycle may be the work of an unknown "Master of the St. Francis legend".1
• Chronological Concerns: If the Assisi frescoes were painted around 1296–1300 and the Arena Chapel around 1303–1305, the interval is considered too short to account for the significant stylistic evolution observed between the two cycles.1
• Attribution Skepticism: Some scholars argue that the works referred to in early historical accounts have perished, and that later writers mistakenly attributed the existing St. Francis cycle to Giotto.1

Conversely, proponents of the attribution suggest that a greater time gap between the two cycles—or Giotto’s experiences in Rome—could explain these stylistic differences.1