The bug

The NBT tag of mobs DeathLootTableSeed can only affect the loot when the mob actually dies. However when you use "/loot kill" command on the mob, the loot is still random instead of fixed.

How to reproduce

1. Summon a sheep with a certain loot table seed:

   /summon minecraft:sheep ~ ~ ~ {DeathLootTable:"minecraft:entities/sheep",DeathLootTableSeed:123}

2. Kill it:

   /kill @e[sort=nearest,type=sheep,limit=1]
   

   → You will get exactly 2 piece of mutton.

3. Summon the sheep again and use /loot command on it:

   /loot give @s kill @e[sort=nearest,type=sheep,limit=1]
   

   → You know how many mutton you get

Code Analysis

Code analysis by clovee can be found in this comment