Have you asked for help in discord?

YES

Describe your desired feature.

I supply some chests with free loot, such as diamond gear and free enchanted books. I want players to "hunt" for the good enchants so it takes a little longer to find books/loot they want.

This might be possible with a custom rule, but I asked in the Discord and it seems they are deterministic.

Example image (before scramble):

Example image (after scramble):

solution found: under "Gui Settings" set Post Action to Shuffle