I think the reason Nightsworn's Lunge (its id is "gr_lunge_p2" if read the code correctly) adds Stealth unconditionally is that you had the performer_effects as 'add_1_stealth', which just adds 1 stealth on skill cast without any condition. I can suggest 'on_hit_as_performer_to_performer_effects, <your new effect>' to achieve your design
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So for now, I just have it give unconditional stealth until I figure it out.
Before:
https://prnt.sc/A_mgqj0YlkhE
After:
https://prnt.sc/N8DWTbE7p9WS