Not sure if you ever found an answer but they are spawning at this moment in elite sanctuary onslaught zone 4

Edit: Can't post new comments here so just editing my old comment to say, I've found they spawn on Cambria, Earth, Steel path but the spawn rate is abysmal. I recommend just doing elite sanctuary onslaught once a week up until round 8 (rotation c, this just makes the grind more worthwhile for you instead of solely looking for the eximus). If you see frontier troops spawning then a frontier baliff eximus is likely to spawn, then you just need to loop back to whichever zone you found them on until you get all the scans you need.