فَإِنْ سَقَطَتْ مِنْ غَيْرِ بُرْءٍ لَمْ يَبْطُلِ الْمَسْحُ، وَإِنْ سَقَطَتْ عَنْ بُرْءٍ بَطَلَ.
Meaning: If the dressing falls off before healing, the wiping remains valid; if it falls because the wound has healed, the wiping concession ends.
A dressing becoming loose before healing does not erase the underlying medical need. If the wound has actually healed, the reason for using a substitute wiping measure ends.
The decisive fact is healing status, not the mere physical act of the dressing falling off.