There are a few decorations drops from some specific flashpoints that seem to have an ultra rare drop chance, IF they even still drop at all, since it is hard to confirm whether some of these still drop at all.

Illumination Probe Crate (Blood Hunt) - my group just finished our 25th run of MM blood hunt including bonus, and still havent seen this drop ever. Reports seem to vary with some claiming they still saw it drop.

Manned Blaster Turret (Colicoid War Games) - you guys fixed the loot drops from the loot crates, but it seems this deco still doesnt drop, seems based on reports from other players it did used to drop many years ago.

Many of the other decorations from older flashpoints that do not have a story mode also seem to have quite low drop rates. Like the Large Field Generator (Kaon Under Sisge), Cryogenics Tank (Lost Island), Starship Medical Tank (Mandalorian Raiders), Banner: Czerka Sunrise (Czerka Corporate Labs), Czerka Data Core (Czerka Core Meltdown). All of these I have seen drop, but only rarely. The uprising mounts similarly I havent seen personally drop since the uprising loot changes adding loot to every uprising boss, before that change I actually saw a couple drop and even got a few myself.

Might be worth checking if some of these drop rates are working as intended, or maybe even increase drop rates, especially since the newer flashpoints drop not only a bigger variety of decorations, but decorations more often, I have seen Spirit of Vengeance and Secrets of the Enclave bosses sometimes drop 2-3 decos at once on Master Mode.