After having played a bit of Tyranids, and confirming that this is indeed the case, it appears that ONLY the Tyranid hiveship counts as synaptic control for the purposes of morale.
While this has lead to some amusing situations, such as a second or even third hiveship losing its morale and fleeing the battlefield, I think it's pretty obvious as to why this is a bit of an issue in both a gameplay and lore sense.
Firstly, lore-wise, Tyranid fleets aren't really supposed to have a singular leading synapse ship. Yes, hiveships are very important and killing them will disrupt the fleet, but there's going to be a lot of hiveships. Their entire point is that each hiveship is the strongest link to the gestalt hivemind there is, forming a distributed command of the entire fleet.
Gameplay wise, this makes Tyranids exceedingly vulnerable to admiral sniping, considering their absurdly low bravery, as well as the fact that their hiveships are rather fragile compared to most other BS', as well as being fairly slow and ponderous (barring the Rush ability) overall! Killing the Admiral hiveship early on may as well be a GG as soon as it happens, since it's all but assured that the entirety (or 90%) of the remaining hivefleet is going to instantly mutiny and run away.
Personally, I'd expect for there to be some benefit to taking multiple hiveships for redundancy, or even BC's with their fleet synapse upgrade. There are a few ways I see this could be done, and I'm curious as to what the forum as a whole thinks:
Every synapse ship provides global synapse control, and every synapse ship must be destroyed in order to cause Tyranid ships to flee.
Every synapse ship provides global synapse control, and every synapse ship lost out of the original synapse amount causes a penalty to some kind of the fleet, with the final synapse ship lost causing mutiny.
Tyranid ships naturally regenerate absurd amounts of morale, but the rate of this is dependent on the proportion of synapse ships currently alive globally, with every synapse ship being destroyed resulting in a 0% rate of regain, and thus inevitable mutiny considering their low bravery.
Tyranid synapse ships provide absolute control as the admiral ship currently does, but only in some finite radius. Tyranid ships quickly lose morale when outside of this radius. Perhaps have this radius be larger for the admiral hiveship, and also upgraded by the BC synapse upgrade? As an example with random numbers pulled entirely from the ether:
Normally, hiveships have 13.5kkm synapse, and the admiral ship gets +4.5kkm synapse range
Upgraded, devourer's have 9kkm synapse, hiveships have 18kkm synapse, and the admiral ship gets +4.5kkm synapse range
I'm sure there are many other solutions, but those 4 are the ones that stuck out to me most immediately.
Personally, I'm partial to the 4th option, along with a significant decrease in price to Tyranid escorts to really drive that "massive swarms of droneships clouding around massive hiveships" idea from fluff home.
What does the forum think of the problem as presented overall, and the solutions proposed?