Even if you use Treachery, when Fade from Treachery proced - it replace any present assa buff. Treachery proc replace not only BoS but even Fade (if you cast it from skill tab).
Only one assa buff at time. It was same since LoD release. Northing was changed in 2.4
PS: Probably TS use some mods thats stop working after client update.