Hello. I will try to answer in order.

I am playing on a desktop. This issue only occurs in Final Fantasy XIV to the best of my knowledge. Changing characters does not resolve the issue.

The issue is no longer present in borderless windowed mode. It is also not present in windowed mode. It only persists in fullscreen.

Switching the mouse to different ports has no effect, and all windows mouse settings are at their default states.

I did notice however that using the OS standard cursor and attempting to directly change the size of the windows cursor through the windows settings exhibited the same behaviour as using the FFXIV custom cursor; if the cursor size is at the default value, it can be seen in FFXIV, but if I attempted to make it bigger, it would also become invisible in-game. I am unsure if this is fully relevant, but it may be useful to know.

I hope this information is useful.