BDO just today (May 17, 2023) fixed the bug on Equilibrium Break that was not applying the bound effect for Witch.  However, there is a similar problem on Fissure Wave that needs to be fixed.  It is supposed to have a knockdown effect, but I have never seen any monster or player knocked down with it in pve or pvp, regardless of levels.  Here is the skill description that says knockdown on hits:

Attack damage 1137% x1, max 10 hits
Critical Hit Rate +50% (PvE only)
Accuracy Rate +15%Maximum 10 targets
Recover 50 MP per hit
Attack Speed -10% for 10 sec on hits
Casting Speed -10% for 10 sec on hits
Knockdown on hits
Down Attack
Damage -52% in PvP only.

Do note that your linked version is the rank 3 of the ability. If that one isn't working, I am pressing you to go for the rank 4 of the skill which is working perfectly fine as intended.

I cannot test the rank 3 of Fissure Wave myself however. The latter is pressing me to "Reset connected skills first" which doesn't work even with Earth's Calling disconnected.

But, to alleviate the issues you're facing permanently, (if you actually such of an issue).  Go MAX rank of the skill (rank 4) and it does its knockdown in both PvP and PvE (granted that the monster can get knockdown).

Despins, lvl 66 awakened witch with 395 ap and accuracy modifers on and my lvl 4 fissure wave NEVER knocks ANYTHING down in pve or pvp.  Please post vid of yours working.  Gracias.

It is working fine for me. I am not sure what you are experimenting, sorry.

https://streamable.com/2ymec6
https://streamable.com/kqw4kl
https://streamable.com/1yxwxd

Thanks very much for the effort of doing this for me.  I am going to keep trying and see what's up. Gracias!

If you keep experiencing the issue when doing the same thing, it may be a good idea to send a ticket to the GMs so they can investigate your account in case something has happened locally.