It is this item: Scoundrel's Leathers - Item - Diablo 4 PTR
“Your Trap and Grenade Skills are also considered Core Skills”
This Inherent Affix does not work, gives no core tag to trap or grenade skills and therefore they also can not trigger core proc effects.
Tested with Aspect of the Expectant
“While you have unlimited Energy from Inner Sight, casting a Core Skill has a 80% chance to spawn Caltrops, Poison Trap, or Death Trap. Gain 20% Core Skill Damage.”
If Caltrops has 0 charges then Soundrels Leathers will no longer work with this skill and falls back to rotating between Death trap and Poison trap only. And if Caltrops is not equipped in the skill bar, it will never spawn with Soundrels Leathers.
This cooldown related limitation doesnt exist for the other two skills and should be fixed.