I've made a copy of my project, deleted all plugins except 2 of yours (log and Battle UI). The same thing occurs (so its safe to say Ive localized the issue). Though youre right I should have done that first and clarified. I'd send that to you via protonmail but there's a 2O~25MB limit for free users.
As for the item, its just a simple battle-item. The dmg formula doesnt matter in this case: Target: 1 Enemy. Consumed on use. HitType: Certain Hit. Load a sample project, make a dummy event with a battle processing command and test the issue there. Just in case: It doesnt occur on self-use items (like potions). Just on offensive items targeting 1 enemy.