A quick glance at TreasureChestCommand.java shows it uses a CommonChestGenerator always; this generator only ever adds common and uncommon loot via its buildLootTableList method which is ultimately called to build the map of allowed tables.

Treasure2 version: Treasure2-mc1.12.2-f14.23.5.2768-v1.1.0.jar
Lib version: GottschCore-mc1.12.2-f14.23.5.2768-v1.5.1.jar

Fixed for v1.2.0 - uses the rarity specified, or COMMON if not specified.