The reason the F-35 was able to return
Iran's SAM exhibit a well known flaw in their proximity fuses.
When the aspect angle is large, the fuse detonation is delayed, causing the shrapnel net to spread after the target has passed.
If the fuse had detonated just a little earlier, the target could have been shot down, but because proximity fuses react more easily to large wings than to aircraft with less reflectivity, the timing is slightly delayed.
In engagements where relative speeds reach Mach 2-3, this slight delay becomes fatal, causing the target to escape even when it appears to be a direct hit.
Videos of this kind have been repeatedly seen in Yemen.
When a proximity fuse detonates in the lower rear of the aircraft, the vital parts of the fighter jet, such as the cockpit and engine intake, are in a blind spot and remain unaffected.
Furthermore, based on combat experience showing that fighter jets are overwhelmingly hit from below by SAMs and anti-aircraft guns, the lower part of the airframe is reinforced.
Therefore, even what appears to be a direct hit can result in minimal damage, allowing the aircraft to return safely. The Yemeni army has narrowly missed capturing Saudi Arabian fighter jets numerous times. This proximity fuse issue is clearly an area that Iran should focus all its efforts on improving.