Sentinel No. 5's outer armor was not especially sturdy, and the "immune cells" breached it. The hideous, grotesque monsters burrowed into the hull and detonated the ammunition depot.
No. 7 was destroyed after being targeted by nearly a hundred monsters. Their sustained bombardment from long range landed almost entirely along a single transverse line, cutting the entire warship in half.
No. 12 had the worst luck. It wasn't brought down by the enemy, but sustained bombardment caused irreversible damage to its hull structure. Displaced energy conduits caused the cooling and simulated-gravity systems to fail, which then led to a capacitor explosion that snapped the ship's main keel. The warship struggled for several minutes, but there was no saving it. It soon fell silent, becoming space debris drifting in place.
Worse still, their enemies weren't another country's warships, but a swarm of giant cells that acted on instinct.
If they had been fighting another country, then under some of the basic conventions of the Galactic Civilization Council, even if a warship was reduced to irretrievable space debris, any crew still alive aboard would at least have a chance of being rescued. During battle, enemy combat units rarely attacked rescue teams. Even pirates, unscrupulous as they were, often observed this tradition.
It was a matter of treating others as you wished to be treated.
But cells didn't need to consider honor among warriors or social niceties. They didn't even have the capacity to "consider" anything.
These things looked like bizarre, grotesque eldritch demons. With cold efficiency, they tore through the armor of the silent ships and slaughtered the surviving crew inside, with no restraint, until their time of activity ran out.
Only later was it confirmed that not one of the more than two thousand crew members aboard the three arsenal ships