This is by far the biggest bug in Homeworld Remastered, and it has been discussed in many other threads and places. Its very sad to see fighter formations rendered mostly useless by this bug, especially after a year of hard work by Gearbox to bring back homeworld 1 style formations.
Each ship in a fighter formation/squadron is picking its own targets. This often results in only the leader firing:
Video: https://youtu.be/OWsI_mgw2xc
To fix the bug, fighters in a formation just need to be bound to their leader’s target. The community is still hoping Gearbox will eventually properly patch this bug.
666 Fix
UPDATE: As of October 2017, the modding community (Trebic aka @shadowwinterknig) has now developed a fix for the bug here. The fix is now part of the 2.3 Players Patch.
Ships Affected
The bug affects fighters in a formation/squadron. Most fighters are affected as they must fire directly in front of themselves:
Hiigaran: Scouts, Interceptors, Bombers
Vaygr: Scouts, Assault Craft, Bombers, Lance, Laser Corvettes
Kushan/Taiidan: Scouts, Interceptors, Bombers, Cloaked Fighters
Corvettes in a formation/squadron do not appear to be impacted much, as they typically have turrets and can fire to the sides:
Hiigaran: Gunship Corvettes, Pulsar Corvettes
Vaygr: Missile Corvettes
Kushan/Taiidan: Light Corvettes, Multigun Corvettes, Heavy Corvettes, Defenders
Note that its often OK, or even beneficial for a formation of corvettes, frigates, capital ships, etc. to pick their own targets.
Dev Comments on #666:
Timeline:
Number 666 first appeared sometime between March 16th, 2016 and April 15th, 2016 in the Testing Patch Beta - and carried on into the 2.0 and 2.1 Patches.
How to Reproduce:
Just have 70 hiigaran interceptors attack 70 kushan interceptors. Follow one hiigaran interceptor squadron closely. After a while only the squadron leader will fire its guns. The bug doesn’t happen every time, but it happens most of the time.
Balance Impact:
hw1 interceptors were intended to be a bit better than hw2 interceptors when in formation, and a bit worse when not in formation. However this wingmen attack bug is wrecking havok with interceptor balance:
70 hiig ints (delta of 5) vs 70 kus ints (no formations) = 45 kus ints in 116 seconds (10+ tests)
70 Hiig Ints (delta of 5) vs 70 kus ints (claws of 5) = 37 hiig ints in 84 seconds (10+ tests)
Suggested changes:
Fix the bug. Then restore proper interceptor balance:
70 hiig ints (delta of 5) vs 70 kus ints (no formations) = hiig ints win by a little
70 Hiig Ints (delta of 5) vs 70 kus ints (claws of 5) = kus ints win by a little
)
so far, everything in the patch preview is perfect, the only thing that is causing trouble that we all see is the 

which is slowly becoming quite unsettling to be honest.
and I agree with you, just wanted to clarify and elaborate a bit on your point.