I'm still on 4b so if this has already been addressed, I apologize for not finding its mention or upgrading to the newer version.
Anyway, it's similar to my previous issues with air-sign Kobolds entering sneak/ambush mode and never leaving it, eventually becoming Legendary+5 thieves/ambushers but significantly slower than all my other 'bolds. Now I'm having the issue with my current fortress of dwarves. If I had to hazard a guess, I ended up with a tier-2 watermancer in my first wave of immigrants and I wonder if her "retreat spell" which she has cast on a couple of occasions has affected those around her who subsequently lack the "spell" that turns the sneak-mode off. Net effect is that I have a dozen or so dwarves who are legendary ambushers who sneak slowly around my fort -- not game-breaking in the least, but a little frustrating at times.
Is there a reaction or a DFHack code I can use to get them out of this mode? I've not tried it yet, but would assigning them to hunt and cancelling it pop them out of their sneakiness?
(PS. Bandit's War Giant Lions have typically shown up as Opposed to Life in 4b -- doesn't break my heart, but I'm sure it's not intended.)