What most directly distinguishes the wāw in تَعْلَمُونَ from the wāw in سَوْفَ?

Al-ʿAṣr and At-Takāthur: Applied Ghunnah, Madd, Lām and Waqf

• Both have identical necessary madd
• Both undergo qalqalah
• Both have nūn sākinah
• Preceding ḍammah makes madd; fatḥah makes līn