The design of Auto Chess faces one fundamental problem: with each new unit the game developers introduce, they make it a little bit harder to find specific units in the shop (i.e. to upgrade your army). The reason is that the shop has a preset percentage chance to roll a certain cost unit for each courier level. Once you roll the unit cost, all units of that cost get an equal chance to appear in that spot.

E.G. if you roll a 5 Gold unit in the shop, and there are 5 legendary units in the game, each one has 20% chance to appear. If there are 6 legendary units in the game, however, their individual chance falls to 16.6%.

This rule is true for all units, but it has the strongest effect for 5-cost units because they are fewest, and each additional unit (in the case of this patch - Sven) changes the chances significantly.