shared custody w my ex and im in the lifestyle now w my new partner. lawyer says one thing reddit says another

ok this is the post i have been afraid to write. m41, divorced from my ex (f40) for 3 yrs, two kids 9 and 12, joint legal joint physical 50/50. ex and i co-parent well, no court drama, mediated divorce, we still text abt school stuff. clean. im now w my partner of 18 months. she is amazing. she had been lifestyle on her own pre us. we have been ENM together since month 6, only after both of us were sure. we have a small set of couples we see, very curated, never at my place when the kids are there (kid weekends are sacred, no phone profiles even open). all play happens at our partner couples place or short hotel stays on her weekends w her kids gone too. now my ex started dating someone serious. and last weekend at kid handoff she said something that froze me — "his friend mentioned your girlfriend was at [a venue]". the venue is a known lifestyle adjacent place locally. it might be nothing. she said it neutrally. but my brain went into a 6 hour spiral on the drive home. i talked to a family lawyer monday. he said in our state ENM by itself is NOT grounds to modify custody but if the ex can show the kids were exposed (kids in the home during play, materials around, partner overstaying when kids r there) it CAN be used as a "best interest" argument. and judges vary wildly. he basically said "you havent done anything legally wrong but you have created risk if she wants to use it". reddit/lifestyle ppl ive read mostly say "youre fine as long as the kids dont know and arent exposed, dont volunteer info, ENM is increasingly normalized in courts". my lawyer said the opposite essentially — "in a contested setting it can be weaponized regardless of what is "normal" elsewhere". so here is the actual question.. do i preemptively tell my ex. not the details. just "for transparency, my partner and i are in an open style relationship, the kids have never been and will never be involved or exposed, i wanted you to hear it from me rather than from her boyfriends friend". the lawyer leaned toward DO tell her in a documented way (text, not call) bc it removes the "he hid it" angle if it ever escalates. but my gut says i am literally handing her ammunition. oh and the other layer btw, the 12 yr old is asking sharper questions lately about who [partner] sees on weekends and stuff. shes not stupid. that timeline is its own problem. anyone been through coming out to your ex when custody is in play. what did you actually do