The only reason that gesturing of any kind in relation to a downed opponent is a problem is because XIV's PvP is not skill based (just like the rest of the game), nor is it in any way a fair contest. And whilst they will never acknowledge PvP's systemic flaw publicly, they do punish actions on KO'd opponents on that basis.

Having someone celebrate or gloat in any manner because you were KO'd in what is almost entirely a luck-based mode, is the cause of it being annoying. It's entirely possible for 'the gloater' to not even realise that they may have contributed as little as 4% of damage to a KO, amongst 3 of their other teammates. Even in 1v1, the PvP stuff is just too fast of a skill spam to be respectably quantifiable.

No one ever complained in CSGO of such things, because that was a quantifiable game with a set structure (rip 2000-2017). You can try and say 1v1 is fair between 2 of the same job- but positioning doesn't count for anything. You're basically counting on the other person to make a mistake and press a button at the wrong time, do it too slow, or forget altogether. There's no skill ceiling that enables you to do something *better* than someone else. It's either 1 or 0 in XIV, you did something or you didn't. There is no capacity for how and out-howing someone else. And thus having someone emote or jump on you when they didn't out-how you, is annoying. They are celebrating that you didn't press a button at a particular point, akin to celebrating you causing a raid to wipe because you missed a tiny bit of dps or something.