I think it’s a graphical bug, but the Aftershock node increases the duration on Fissure by 1-5 seconds. I’ve tried it on the training dummy and it does appear to continue doing damage for ~5 seconds after the initial hit but the graphic for Fissure disappears after ~2 seconds (despite having +5 second duration extension from Aftershock).
Additionally, the Fissue node tooltip doesn’t say how long the Fissure lasts (presumably ~2 seconds given the graphical effect).
However, if the Descend adds a 5s cooldown (which it does seem to), the damage from Fissure/Aftershock finishes maybe 1 second before the cooldown from Descend finishes which would mean that the max duration on Fissure/Aftershock can only be ~4 seconds, not the 5-7 seconds that it should logically be.
TLDR: The graphical effect from Fissure finishes too early if you have points in Aftershock node and the damage from Fissure (with +5 sec duration from maxxed Aftershock) should outlast the Descend cooldown but doesn’t (by what appears to be ~1 second & the Descend cooldown does appear to be 5 seconds).