When you first login having equipped the blade they seem to work well and seaths are on your back. There are some clipping issues with most outfits but the seaths in specific are fine, however when you start combat the seaths disappear after using a few skills and despite many think they disappear, they dont. They actually follow you on the floor behind and almost attached to your feet(which is why many think they disappear, cause they arent so visible there on rough terrains). I hope this helps on reproducing it
Edit
I checked on the toon I have dual ones 5 mins ago and he didnt seem to trigger. the bug It may had been fixed, but will check further
Edit 2
Nope, not fully fixed, now it happened after a cutscene with the Warden in beltsavis and not during combat:
https://imgur.com/sCPam1r
You can see the seaths on the floor behind my feet