I honestly couldn't tell you why you are only seeing it in FFXIV, but I wanted to let ya know that this is a pretty common issue, and I used to suffer from it. It affected almost all of my games, some in more obvious ways (maybe that what it is).

The issue is caused by having the different refresh rates, basically what happens is when your GPU is asked to render something on a monitor it limits all rendering to that refresh rate. So if you have a 60hz Monitor and a 144hz monitor, and you do anything GPU rendered on the 60hz, both monitors will start rendering at 60hz. I ended up buying a second 144hz monitor to resolve the problem for myself.

Sidenote: I'm not sure how 100% accurate the above is, its just what I was able to find over the year or so I suffered a similar issue in games other than FFXIV.