When multiple fleets attack a planet, planetary defenses will only engage one or possibly two of them at a time.
Example: a planet with hypersonic missiles was attacked by an all-starfrigate fleet and an explorer fleet. At the start of the battle the hypersonic missiles opened up against the starfrigate fleet and continued firing at them throughout the battle. The starfrigate fleet was in high orbit and the hypersonics on the opposite side of the world from the fleet at any give time couldn't reach them and weren't firing
Meanwhile the explorers closed to extremely short range and opened up, but the far-side hypersonic missiles did not engage them at all, even when they were on the opposite side of the planet from the starfrigates (where the batteries weren't firing at anything).
This ties into the general problem of opportunity fire (https://ministry.kronosaur.com/record.hexm?id=61213) but it is especially egregious for defenses since all defenses are in one "wing" or combat group.
Contrary to what domina says below this does affect hypersonics, provided that the ships are in different fleets. domina was responding to an earlier version of this bug report that expressed it as a problem of wings, rather than of fleets.