Ok glad someone else is seeing it, though it doesn't seem to be super common as not seeing reports everywhere about it. As you said orion its very spotty. Most of he time it works fine, but sometimes it just flat doesn't. Seems really random too in how it works. Have played about 4-5 games since I posted this originally, and have seen the bug pop in and out which might be why people have trouble detecting it. As an example have had my capitals fire, and my assaults escorts and guardians just fly in circles with the out of range bug despite some of those being missle boats, the ships that weren't firing did nothing until all the ships that were firing died then they engaged. Have seen just one class of ship (random which one) not fight, while the others fight. Have even seen it affects starbase encounters where the starbase won't shoot or be shot at for a prolonged period of time. Eventually the game sorta reset itsself i guess and the 2 sides fought. I get the feeling this bug isn't super uncommon... but because of the nature of how it works unless you are paying close attention, when it DOES occur, you can easily miss it. It is just at its most noticeable when you have 6 capitals/assaults/escorts circling the enemy while they watch the entire opposing fleet fight the drones.
As an aside I believe I have an educated guess on the cause after seeing this enough times now. I believe there is supposed to be a script fired off that signals all ships to engage, and then there is a redundancy back up that fires off again for ships to fight when they are triggered via their or the enemies attack/defense priorities. IE, it would look like this. Interceptors are positioned forward of the fleet (this is what the one of the drone specilization carriers gives you) They will always get shot at first regardless of targeting priorities by enemy ships, this is because they are always in range of the enemy at the start of the fight. No ship has a priority to defend interceptors, so its possible that they don't triggered, the same happens for assaults, though ships do have priorities to attack them.
Where this becomes important is the guardians escorts and capitals. A capital has no assist priorities, which makes them one of the most likely to fly around doing nothing, this causes the guardian to also be a prime culprit. Escorts are set to defend capitals, but capitals will never be attacked until escorts are down, so they will be triggered generally right after the assaults, as they are the next target. Once escorts are down capitals are typically the next target, this will trigger your guardians, as well as they have a defense assist trigger for capitals. So if your capitals attack guardians will always fight too.
The chain mentioned above can break at any of the links above, or not break at all. So fights can appear to go completely fine one time, and the next your capitals fly in circles with (out of range) while the rest of the fleet fights. If you say only have 1 ship tagged as a capital and 6 ships that aren't its very possible not to notice that one of your ships isn't fighting. I have noticed this bug doesn't seem to affect the AI and I believe the reason for this is the AI only uses two classes of ships generally. They only produce escorts or supports, so as there is only 1 possible trigger to occur, in their fleets, they will always function appropriately. As the game will always successfully activate the first link in the chain on both sides. I get the feeling this will be a very hard bug to track down, hope they can manage it.