It's not actually a bug, but I guess it's not normal for the targeted design in the caravans system. I studies COST/GAINS for the different crates in the caravan system.
All case have the same rentability GLINT/MONEY (1,3 sylver per white glint for this travel).
But if you wanna optimize space in your caravans, uncommon cases are by far the best ATM, so for people that have lot of glints, rentability is better if you fullfill your caravan with Case Uncommon.
Find in attachment my calculations and an visual example :
